From 31c1e5943fdb2608089c8268cdb7261942e247ac Mon Sep 17 00:00:00 2001
From: Xinyuan Tong <115166877+JustinTong0323@users.noreply.github.com>
Date: Thu, 6 Aug 2026 15:34:31 +0800
Subject: [PATCH] Facade DSA index-cache: MTP topk-reuse state + index-K
 storage (#28609)

---
 .../srt/layers/attention/index_topk_share.py  |  86 ++++++
 .../srt/mem_cache/dsa_cache_layer_split.py    | 282 +++++++++---------
 .../sglang/srt/mem_cache/index_key_cache.py   | 149 +++++++++
 python/sglang/srt/mem_cache/memory_pool.py    | 145 ++-------
 python/sglang/srt/models/deepseek_nextn.py    |  33 +-
 python/sglang/srt/models/deepseek_v2.py       |  14 +-
 .../sglang/srt/speculative/eagle_worker_v2.py | 182 ++++++-----
 .../layers/attention/test_index_topk_share.py | 163 ++++++++++
 .../test_dsa_layer_split_broadcast.py         |   6 +-
 9 files changed, 672 insertions(+), 388 deletions(-)
 create mode 100644 python/sglang/srt/layers/attention/index_topk_share.py
 create mode 100644 python/sglang/srt/mem_cache/index_key_cache.py
 create mode 100644 test/registered/unit/layers/attention/test_index_topk_share.py

diff --git a/python/sglang/srt/layers/attention/index_topk_share.py b/python/sglang/srt/layers/attention/index_topk_share.py
new file mode 100644
index 000000000..e7500d7da
--- /dev/null
+++ b/python/sglang/srt/layers/attention/index_topk_share.py
@@ -0,0 +1,86 @@
+from __future__ import annotations
+
+from contextlib import contextmanager
+from typing import TYPE_CHECKING, Iterator, Optional
+
+if TYPE_CHECKING:
+    import torch
+
+    from sglang.srt.model_executor.forward_batch_info import ForwardBatch
+
+
+class IndexTopKShareState:
+    def __init__(
+        self,
+        forward_batch: ForwardBatch,
+        topk_indices: Optional[torch.Tensor],
+    ):
+        self._forward_batch = forward_batch
+        self._topk_indices = topk_indices
+
+    @classmethod
+    def from_mtp_carry(cls, forward_batch: ForwardBatch) -> IndexTopKShareState:
+        topk_indices = (
+            forward_batch.spec_info.dsa_topk_indices
+            if forward_batch.reuse_dsa_topk_indices
+            else None
+        )
+        return cls(forward_batch, topk_indices)
+
+    @property
+    def enabled(self) -> bool:
+        return bool(self._forward_batch.reuse_dsa_topk_indices)
+
+    @property
+    def _seed_buf(self) -> Optional[torch.Tensor]:
+        if self._forward_batch.forward_mode.is_extend(include_draft_extend_v2=True):
+            return self._forward_batch.spec_info.dsa_seed_topk_capture
+        return None
+
+    @property
+    def should_publish(self) -> bool:
+        return self.enabled or self._seed_buf is not None
+
+    @property
+    def topk_indices(self) -> Optional[torch.Tensor]:
+        return self._topk_indices
+
+    def update(self, topk_indices: Optional[torch.Tensor]) -> None:
+        self._topk_indices = topk_indices
+
+    def publish(self) -> None:
+        if self._topk_indices is None or not self.should_publish:
+            return
+        if self.enabled:
+            self._forward_batch.spec_info.dsa_topk_indices = self._topk_indices
+        seed_buf = self._seed_buf
+        if seed_buf is not None:
+            sel = self._forward_batch.spec_info.dsa_seed_topk_select
+            src = (
+                self._topk_indices[: seed_buf.shape[0]]
+                if sel is None
+                else self._topk_indices[sel]
+            )
+            seed_buf[: src.shape[0]].copy_(src)
+
+    @classmethod
+    @contextmanager
+    def mtp_iteration(
+        cls,
+        forward_batch: ForwardBatch,
+        enabled: bool = True,
+        keep_carry_seed: bool = False,
+    ) -> Iterator[Optional[IndexTopKShareState]]:
+        if not enabled:
+            yield None
+            return
+        spec_info = forward_batch.spec_info
+        forward_batch.reuse_dsa_topk_indices = True
+        # Keep the draft-extend seed so step 0 reuses it; else recompute it.
+        if not (keep_carry_seed and spec_info.dsa_topk_indices is not None):
+            spec_info.dsa_topk_indices = None
+        try:
+            yield cls.from_mtp_carry(forward_batch)
+        finally:
+            spec_info.dsa_topk_indices = None
+            forward_batch.reuse_dsa_topk_indices = False
diff --git a/python/sglang/srt/mem_cache/dsa_cache_layer_split.py b/python/sglang/srt/mem_cache/dsa_cache_layer_split.py
index 75f12030a..5915d7639 100644
--- a/python/sglang/srt/mem_cache/dsa_cache_layer_split.py
+++ b/python/sglang/srt/mem_cache/dsa_cache_layer_split.py
@@ -37,6 +37,7 @@ import torch
 
 from sglang.kernels.ops.attention.dsa import index_buf_accessor
 from sglang.srt.layers.cp.utils import get_layer_owner, get_layer_shard_range
+from sglang.srt.mem_cache.index_key_cache import IndexKeyCache
 from sglang.srt.mem_cache.memory_pool import (
     GPU_MEMORY_TYPE_KV_CACHE,
     DSATokenToKVPool,
@@ -53,6 +54,141 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
+class LayerSplitIndexKeyCache(IndexKeyCache):
+    def __init__(self, pool: LayerSplitDSATokenToKVPool, index_buf_size: int):
+        super().__init__(pool, index_buf_size)
+        num_pages = (index_buf_size + pool.page_size + 1) // pool.page_size
+        with (
+            torch.cuda.use_mem_pool(pool.custom_mem_pool)
+            if pool.custom_mem_pool
+            else nullcontext()
+        ):
+            self.remote_buffer = torch.empty(
+                self._buffer_shape(num_pages),
+                dtype=pool.index_k_with_scale_buffer_dtype,
+                device=pool.device,
+            )
+        self.remote_layer_id: Optional[int] = None
+
+    def _layer_num_pages(self, layer_idx: int, num_pages: int) -> int:
+        layer_id = self.pool.start_layer + layer_idx
+        return num_pages if self.pool._is_layer_owned(layer_id) else 0
+
+    def clear(self) -> None:
+        super().clear()
+        del self.remote_buffer
+
+    def move(self, tgt_loc: torch.Tensor, src_loc: torch.Tensor) -> None:
+        if tgt_loc.numel() == 0:
+            return
+        tgt_loc_flat = tgt_loc.view(-1).long()
+        src_loc_flat = src_loc.view(-1).long()
+        for index_k in self.buffer:
+            if index_k.shape[0] != 0:
+                index_k[tgt_loc_flat] = index_k[src_loc_flat]
+
+    def get_buffer(self, layer_id: int) -> torch.Tensor:
+        if self.pool.layer_transfer_counter is not None:
+            self.pool.layer_transfer_counter.wait_until(
+                layer_id - self.pool.start_layer
+            )
+        return self.get_broadcastable_buffer(layer_id)
+
+    def get_k_and_scale(
+        self,
+        layer_id: int,
+        seq_len_tensor: torch.Tensor,
+        page_indices: torch.Tensor,
+        seq_len_sum: int,
+        max_seq_len: int,
+    ):
+        buf = self.get_buffer(layer_id)
+        self.pool.prefetch_kv_buffer(layer_id)
+        return index_buf_accessor.GetKAndS.execute(
+            self.pool,
+            buf,
+            page_indices=page_indices,
+            seq_len_tensor=seq_len_tensor,
+            seq_len_sum=seq_len_sum,
+            max_seq_len=max_seq_len,
+        )
+
+    def store_quantized(
+        self,
+        layer_id: int,
+        loc: torch.Tensor,
+        index_k: torch.Tensor,
+        index_k_scale: torch.Tensor,
+    ) -> None:
+        self.invalidate(layer_id)
+        if self.pool._is_layer_owned(layer_id):
+            super().store_quantized(layer_id, loc, index_k, index_k_scale)
+
+    def invalidate(self, layer_id: int) -> None:
+        if self.remote_layer_id == layer_id:
+            self.remote_layer_id = None
+
+    def get_broadcastable_buffer(self, layer_id: int) -> torch.Tensor:
+        if self.remote_layer_id != layer_id:
+            local_idx = layer_id - self.pool.start_layer
+            src_tensor = (
+                self.buffer[local_idx] if self.pool._is_layer_owned(layer_id) else None
+            )
+            self.pool._broadcast_tensor_from_owner(
+                self.remote_buffer,
+                layer_id,
+                src_tensor=src_tensor,
+            )
+            self.remote_layer_id = layer_id
+        return self.remote_buffer
+
+    def state_buf_infos(self):
+        owned_layer_ids = [
+            i
+            for i in range(self.pool.layer_num)
+            if self.pool._is_layer_owned(self.pool.start_layer + i)
+        ]
+        data_ptrs = [self.buffer[i].data_ptr() for i in owned_layer_ids]
+        data_lens = [self.buffer[i].nbytes for i in owned_layer_ids]
+        item_lens = [self.buffer[i][0].nbytes for i in owned_layer_ids]
+        return data_ptrs, data_lens, item_lens
+
+    def cpu_copy(self, indices):
+        page_indices = indices[:: self.pool.page_size] // self.pool.page_size
+        torch.cuda.synchronize()
+        index_k_cpu = []
+        chunk_size = self.pool.cpu_offloading_chunk_size
+        page_chunk_size = max(1, chunk_size // self.pool.page_size)
+        for layer_id in range(self.pool.layer_num):
+            index_k_cpu.append([])
+            if self.buffer[layer_id].shape[0] == 0:
+                continue
+            for i in range(0, len(page_indices), page_chunk_size):
+                chunk_page_indices = page_indices[i : i + page_chunk_size]
+                idx_cpu = self.buffer[layer_id][chunk_page_indices].to(
+                    "cpu", non_blocking=True
+                )
+                index_k_cpu[-1].append(idx_cpu)
+        torch.cuda.synchronize()
+        return index_k_cpu
+
+    def load_cpu_copy(self, index_k_cpu, indices) -> None:
+        page_indices = indices[:: self.pool.page_size] // self.pool.page_size
+        torch.cuda.synchronize()
+        chunk_size = self.pool.cpu_offloading_chunk_size
+        page_chunk_size = max(1, chunk_size // self.pool.page_size)
+        for layer_id in range(self.pool.layer_num):
+            if self.buffer[layer_id].shape[0] == 0:
+                continue
+            for i in range(0, len(page_indices), page_chunk_size):
+                chunk_page_indices = page_indices[i : i + page_chunk_size]
+                idx_cpu = index_k_cpu[layer_id][i // page_chunk_size]
+                assert idx_cpu.shape[0] == len(chunk_page_indices)
+                idx_chunk = idx_cpu.to(self.buffer[layer_id].device, non_blocking=True)
+                self.buffer[layer_id][chunk_page_indices] = idx_chunk
+        torch.cuda.synchronize()
+
+
 class LayerSplitDSATokenToKVPool(DSATokenToKVPool):
     """DSA KV pool that shards layers across CP ranks with owner-broadcast reads."""
 
@@ -203,35 +339,13 @@ class LayerSplitDSATokenToKVPool(DSATokenToKVPool):
                 self.pending_remote_kv_broadcast = False
         self._init_layer_broadcast_comm()
 
-    def _create_index_buffers(self):
-        num_pages = (self.index_buf_size + self.page_size + 1) // self.page_size
-        with (
-            torch.cuda.use_mem_pool(self.custom_mem_pool)
-            if self.custom_mem_pool
-            else nullcontext()
-        ):
-            self.index_k_with_scale_buffer = [
-                torch.zeros(
-                    self._index_buffer_shape(
-                        num_pages if self._is_layer_owned(self.start_layer + i) else 0
-                    ),
-                    dtype=self.index_k_with_scale_buffer_dtype,
-                    device=self.device,
-                )
-                for i in range(self.layer_num)
-            ]
-            self.remote_index_k_with_scale_buffer = torch.empty(
-                self._index_buffer_shape(num_pages),
-                dtype=self.index_k_with_scale_buffer_dtype,
-                device=self.device,
-            )
-            self.remote_index_layer_id: Optional[int] = None
+    def _create_index_key_cache(self) -> IndexKeyCache:
+        return LayerSplitIndexKeyCache(self, self.index_buf_size)
 
     def _clear_buffers(self):
         del self.kv_buffer
         del self.remote_kv_buffer
-        del self.remote_index_k_with_scale_buffer
-        del self.index_k_with_scale_buffer
+        self.index_key_cache.clear()
 
     # ---- MLA latent KV: owned-only writes, owner-broadcast reads ----------
 
@@ -418,96 +532,20 @@ class LayerSplitDSATokenToKVPool(DSATokenToKVPool):
             if kv_cache.shape[0] == 0:
                 continue
             kv_cache[tgt_loc_flat] = kv_cache[src_loc_flat]
-        for index_k in self.index_k_with_scale_buffer:
-            if index_k.shape[0] == 0:
-                continue
-            index_k[tgt_loc_flat] = index_k[src_loc_flat]
+        self.index_key_cache.move(tgt_loc, src_loc)
 
     # ---- DSA indexer buffer: owned-only writes, owner-broadcast reads -----
 
     def get_broadcastable_index_k_with_scale_buffer(
         self, layer_id: int
     ) -> torch.Tensor:
-        if self.layer_transfer_counter is not None:
-            self.layer_transfer_counter.wait_until(layer_id - self.start_layer)
-        return self._get_broadcastable_index_buffer(layer_id)
-
-    def get_index_k_continuous(self, layer_id, seq_len, page_indices):
-        if self.layer_transfer_counter is not None:
-            self.layer_transfer_counter.wait_until(layer_id - self.start_layer)
-        buf = self._get_broadcastable_index_buffer(layer_id)
-        return index_buf_accessor.GetK.execute(
-            self, buf, seq_len=seq_len, page_indices=page_indices
-        )
-
-    def get_index_k_scale_continuous(self, layer_id, seq_len, page_indices):
-        if self.layer_transfer_counter is not None:
-            self.layer_transfer_counter.wait_until(layer_id - self.start_layer)
-        buf = self._get_broadcastable_index_buffer(layer_id)
-        return index_buf_accessor.GetS.execute(
-            self, buf, seq_len=seq_len, page_indices=page_indices
-        )
-
-    def get_index_k_scale_buffer(
-        self, layer_id, seq_len_tensor, page_indices, seq_len_sum, max_seq_len
-    ):
-        if self.layer_transfer_counter is not None:
-            self.layer_transfer_counter.wait_until(layer_id - self.start_layer)
-        buf = self._get_broadcastable_index_buffer(layer_id)
-        # Overlap the latent-KV owner-broadcast with the indexer read.
-        self.prefetch_kv_buffer(layer_id)
-        return index_buf_accessor.GetKAndS.execute(
-            self,
-            buf,
-            page_indices=page_indices,
-            seq_len_tensor=seq_len_tensor,
-            seq_len_sum=seq_len_sum,
-            max_seq_len=max_seq_len,
-        )
-
-    def set_index_k_scale_buffer(self, layer_id, loc, index_k, index_k_scale) -> None:
-        self.invalidate_index_buffer_for_layer(layer_id)
-        if not self._is_layer_owned(layer_id):
-            return
-        buf = self.index_k_with_scale_buffer[layer_id - self.start_layer]
-        index_buf_accessor.SetKAndS.execute(
-            pool=self, buf=buf, loc=loc, index_k=index_k, index_k_scale=index_k_scale
-        )
+        return self.index_key_cache.get_buffer(layer_id)
 
     def invalidate_index_buffer_for_layer(self, layer_id: int) -> None:
-        if self.remote_index_layer_id == layer_id:
-            self.remote_index_layer_id = None
+        self.index_key_cache.invalidate(layer_id)
 
     def _get_broadcastable_index_buffer(self, layer_id: int) -> torch.Tensor:
-        if self.remote_index_layer_id != layer_id:
-            local_idx = self._local_layer_idx(layer_id)
-            src_tensor = (
-                self.index_k_with_scale_buffer[local_idx]
-                if self._is_layer_owned(layer_id)
-                else None
-            )
-            self._broadcast_tensor_from_owner(
-                self.remote_index_k_with_scale_buffer,
-                layer_id,
-                src_tensor=src_tensor,
-            )
-            self.remote_index_layer_id = layer_id
-        return self.remote_index_k_with_scale_buffer
-
-    def get_state_buf_infos(self):
-        owned_layer_ids = [
-            i
-            for i in range(self.layer_num)
-            if self._is_layer_owned(self.start_layer + i)
-        ]
-        data_ptrs = [
-            self.index_k_with_scale_buffer[i].data_ptr() for i in owned_layer_ids
-        ]
-        data_lens = [self.index_k_with_scale_buffer[i].nbytes for i in owned_layer_ids]
-        item_lens = [
-            self.index_k_with_scale_buffer[i][0].nbytes for i in owned_layer_ids
-        ]
-        return data_ptrs, data_lens, item_lens
+        return self.index_key_cache.get_broadcastable_buffer(layer_id)
 
     # ---- HiCache CPU offload: skip empty (non-owned) layers ---------------
 
@@ -529,22 +567,7 @@ class LayerSplitDSATokenToKVPool(DSATokenToKVPool):
                 kv_cache_cpu[-1].append(kv_cpu)
         current_platform.synchronize()
 
-        page_indices = indices[:: self.page_size] // self.page_size
-        torch.cuda.synchronize()
-        index_k_cpu = []
-        page_chunk_size = max(1, chunk_size // self.page_size)
-        for layer_id in range(self.layer_num):
-            index_k_cpu.append([])
-            if self.index_k_with_scale_buffer[layer_id].shape[0] == 0:
-                continue
-            for i in range(0, len(page_indices), page_chunk_size):
-                chunk_page_indices = page_indices[i : i + page_chunk_size]
-                idx_cpu = self.index_k_with_scale_buffer[layer_id][
-                    chunk_page_indices
-                ].to("cpu", non_blocking=True)
-                index_k_cpu[-1].append(idx_cpu)
-        torch.cuda.synchronize()
-        return {"kv": kv_cache_cpu, "index_k": index_k_cpu}
+        return {"kv": kv_cache_cpu, "index_k": self.index_key_cache.cpu_copy(indices)}
 
     def load_cpu_copy(self, kv_cache_cpu_dict, indices, mamba_indices=None):
         from sglang.srt.utils import current_platform
@@ -563,19 +586,4 @@ class LayerSplitDSATokenToKVPool(DSATokenToKVPool):
                 self.kv_buffer[layer_id][chunk_indices] = kv_chunk
         current_platform.synchronize()
 
-        page_indices = indices[:: self.page_size] // self.page_size
-        index_k_cpu = kv_cache_cpu_dict["index_k"]
-        torch.cuda.synchronize()
-        page_chunk_size = max(1, chunk_size // self.page_size)
-        for layer_id in range(self.layer_num):
-            if self.index_k_with_scale_buffer[layer_id].shape[0] == 0:
-                continue
-            for i in range(0, len(page_indices), page_chunk_size):
-                chunk_page_indices = page_indices[i : i + page_chunk_size]
-                idx_cpu = index_k_cpu[layer_id][i // page_chunk_size]
-                assert idx_cpu.shape[0] == len(chunk_page_indices)
-                idx_chunk = idx_cpu.to(
-                    self.index_k_with_scale_buffer[layer_id].device, non_blocking=True
-                )
-                self.index_k_with_scale_buffer[layer_id][chunk_page_indices] = idx_chunk
-        torch.cuda.synchronize()
+        self.index_key_cache.load_cpu_copy(kv_cache_cpu_dict["index_k"], indices)
diff --git a/python/sglang/srt/mem_cache/index_key_cache.py b/python/sglang/srt/mem_cache/index_key_cache.py
new file mode 100644
index 000000000..18eecec60
--- /dev/null
+++ b/python/sglang/srt/mem_cache/index_key_cache.py
@@ -0,0 +1,149 @@
+from __future__ import annotations
+
+from contextlib import nullcontext
+from typing import TYPE_CHECKING
+
+import torch
+
+from sglang.kernels.ops.attention.dsa import index_buf_accessor
+
+if TYPE_CHECKING:
+    from sglang.srt.mem_cache.memory_pool import DSATokenToKVPool
+
+
+class IndexKeyCache:
+    def __init__(self, pool: DSATokenToKVPool, index_buf_size: int):
+        self.pool = pool
+        num_pages = (index_buf_size + pool.page_size + 1) // pool.page_size
+        with (
+            torch.cuda.use_mem_pool(pool.custom_mem_pool)
+            if pool.custom_mem_pool
+            else nullcontext()
+        ):
+            self.buffer = [
+                torch.zeros(
+                    self._buffer_shape(self._layer_num_pages(i, num_pages)),
+                    dtype=pool.index_k_with_scale_buffer_dtype,
+                    device=pool.device,
+                )
+                for i in range(pool.layer_num)
+            ]
+
+    def _buffer_shape(self, num_pages: int) -> tuple[int, int]:
+        pool = self.pool
+        return (
+            num_pages,
+            pool.page_size
+            * (pool.index_head_dim + pool.index_head_dim // pool.quant_block_size * 4),
+        )
+
+    def _layer_num_pages(self, layer_idx: int, num_pages: int) -> int:
+        return num_pages
+
+    def clear(self) -> None:
+        del self.buffer
+
+    def move(self, tgt_loc: torch.Tensor, src_loc: torch.Tensor) -> None:
+        if tgt_loc.numel() == 0:
+            return
+        tgt_loc_flat = tgt_loc.view(-1).long()
+        src_loc_flat = src_loc.view(-1).long()
+        for index_k in self.buffer:
+            index_k[tgt_loc_flat] = index_k[src_loc_flat]
+
+    def get_local_buffer(self, layer_id: int) -> torch.Tensor:
+        if self.pool.layer_transfer_counter is not None:
+            self.pool.layer_transfer_counter.wait_until(
+                layer_id - self.pool.start_layer
+            )
+        return self.buffer[layer_id - self.pool.start_layer]
+
+    def get_buffer(self, layer_id: int) -> torch.Tensor:
+        return self.get_local_buffer(layer_id)
+
+    def get_k_continuous(self, layer_id: int, seq_len: int, page_indices: torch.Tensor):
+        buf = self.get_buffer(layer_id)
+        return index_buf_accessor.GetK.execute(
+            self.pool, buf, seq_len=seq_len, page_indices=page_indices
+        )
+
+    def get_k_scale_continuous(
+        self, layer_id: int, seq_len: int, page_indices: torch.Tensor
+    ):
+        buf = self.get_buffer(layer_id)
+        return index_buf_accessor.GetS.execute(
+            self.pool, buf, seq_len=seq_len, page_indices=page_indices
+        )
+
+    def get_k_and_scale(
+        self,
+        layer_id: int,
+        seq_len_tensor: torch.Tensor,
+        page_indices: torch.Tensor,
+        seq_len_sum: int,
+        max_seq_len: int,
+    ):
+        buf = self.get_buffer(layer_id)
+        return index_buf_accessor.GetKAndS.execute(
+            self.pool,
+            buf,
+            page_indices=page_indices,
+            seq_len_tensor=seq_len_tensor,
+            seq_len_sum=seq_len_sum,
+            max_seq_len=max_seq_len,
+        )
+
+    def store_quantized(
+        self,
+        layer_id: int,
+        loc: torch.Tensor,
+        index_k: torch.Tensor,
+        index_k_scale: torch.Tensor,
+    ) -> None:
+        buf = self.buffer[layer_id - self.pool.start_layer]
+        index_buf_accessor.SetKAndS.execute(
+            pool=self.pool,
+            buf=buf,
+            loc=loc,
+            index_k=index_k,
+            index_k_scale=index_k_scale,
+        )
+
+    def cpu_copy(self, indices):
+        # Retracted pages may be reused before resume, so offload index-K with KV.
+        page_indices = indices[:: self.pool.page_size] // self.pool.page_size
+        torch.cuda.synchronize()
+        index_k_cpu = []
+        chunk_size = self.pool.cpu_offloading_chunk_size
+        page_chunk_size = max(1, chunk_size // self.pool.page_size)
+        for layer_id in range(self.pool.layer_num):
+            index_k_cpu.append([])
+            for i in range(0, len(page_indices), page_chunk_size):
+                chunk_page_indices = page_indices[i : i + page_chunk_size]
+                idx_cpu = self.buffer[layer_id][chunk_page_indices].to(
+                    "cpu", non_blocking=True
+                )
+                index_k_cpu[-1].append(idx_cpu)
+        torch.cuda.synchronize()
+        return index_k_cpu
+
+    def load_cpu_copy(self, index_k_cpu, indices) -> None:
+        page_indices = indices[:: self.pool.page_size] // self.pool.page_size
+        torch.cuda.synchronize()
+        chunk_size = self.pool.cpu_offloading_chunk_size
+        page_chunk_size = max(1, chunk_size // self.pool.page_size)
+        for layer_id in range(self.pool.layer_num):
+            for i in range(0, len(page_indices), page_chunk_size):
+                chunk_page_indices = page_indices[i : i + page_chunk_size]
+                idx_cpu = index_k_cpu[layer_id][i // page_chunk_size]
+                assert idx_cpu.shape[0] == len(chunk_page_indices)
+                idx_chunk = idx_cpu.to(self.buffer[0].device, non_blocking=True)
+                self.buffer[layer_id][chunk_page_indices] = idx_chunk
+        torch.cuda.synchronize()
+
+    def state_buf_infos(self):
+        layer_num = self.pool.layer_num
+        data_ptrs = [self.buffer[i].data_ptr() for i in range(layer_num)]
+        data_lens = [self.buffer[i].nbytes for i in range(layer_num)]
+        item_lens = [self.buffer[i][0].nbytes for i in range(layer_num)]
+        return data_ptrs, data_lens, item_lens
diff --git a/python/sglang/srt/mem_cache/memory_pool.py b/python/sglang/srt/mem_cache/memory_pool.py
index bcd757288..0aa942076 100644
--- a/python/sglang/srt/mem_cache/memory_pool.py
+++ b/python/sglang/srt/mem_cache/memory_pool.py
@@ -38,7 +38,6 @@ import torch
 import triton
 import triton.language as tl
 
-from sglang.kernels.ops.attention.dsa import index_buf_accessor
 from sglang.kernels.ops.attention.dsa.quant_k_cache import (
     quantize_k_cache,
     quantize_k_cache_separate,
@@ -59,6 +58,7 @@ from sglang.srt.layers.quantization.fp4_kv_cache_quant_method import (
 )
 from sglang.srt.layers.radix_attention import RadixAttention
 from sglang.srt.mem_cache.allocator.mamba import MambaSlotAllocator
+from sglang.srt.mem_cache.index_key_cache import IndexKeyCache
 from sglang.srt.mem_cache.kv_vmm_backing import KvVmmBufferOwner
 from sglang.srt.mem_cache.layout.page_major import (
     build_page_major_mamba_views,
@@ -4378,58 +4378,28 @@ class DSATokenToKVPool(MLATokenToKVPool):
                 ), f"HIP legacy DSA path requires page_size == 1, got {self.page_size}"
         else:
             assert self.page_size == 64
-        self._create_index_buffers()
+        self.index_key_cache = self._create_index_key_cache()
         self._finalize_allocation_log(size)
 
-    def _index_buffer_shape(self, num_pages: int) -> tuple[int, int]:
-        return (
-            num_pages,
-            self.page_size
-            * (self.index_head_dim + self.index_head_dim // self.quant_block_size * 4),
-        )
+    def _create_index_key_cache(self) -> IndexKeyCache:
+        return IndexKeyCache(self, self.index_buf_size)
 
-    def _create_index_buffers(self):
-        num_pages = (self.index_buf_size + self.page_size + 1) // self.page_size
-        with (
-            torch.cuda.use_mem_pool(self.custom_mem_pool)
-            if self.custom_mem_pool
-            else nullcontext()
-        ):
-            self.index_k_with_scale_buffer = [
-                torch.zeros(
-                    # Layout:
-                    #     ref: test_attention.py :: kv_cache_cast_to_fp8
-                    #     shape: (num_pages, page_size 64 * head_dim 128 + page_size 64 * fp32_nbytes 4)
-                    #     data: for page i,
-                    #         * buf[i, :page_size * head_dim] for fp8 data
-                    #         * buf[i, page_size * head_dim:].view(float32) for scale
-                    self._index_buffer_shape(num_pages),
-                    dtype=self.index_k_with_scale_buffer_dtype,
-                    device=self.device,
-                )
-                for _ in range(self.layer_num)
-            ]
+    @property
+    def index_k_with_scale_buffer(self):
+        # Preserve direct HiCache access while storage lives behind the facade.
+        return self.index_key_cache.buffer
 
     def _clear_buffers(self):
         super()._clear_buffers()
-        del self.index_k_with_scale_buffer
+        self.index_key_cache.clear()
 
     def move_kv_cache(self, tgt_loc: torch.Tensor, src_loc: torch.Tensor):
         """Move latent KV and the DSA indexer cache (key + scale) in lockstep."""
         super().move_kv_cache(tgt_loc, src_loc)
-
-        if tgt_loc.numel() == 0:
-            return
-
-        tgt_loc_flat = tgt_loc.view(-1).long()
-        src_loc_flat = src_loc.view(-1).long()
-        for index_k in self.index_k_with_scale_buffer:
-            index_k[tgt_loc_flat] = index_k[src_loc_flat]
+        self.index_key_cache.move(tgt_loc, src_loc)
 
     def get_index_k_with_scale_buffer(self, layer_id: int) -> torch.Tensor:
-        if self.layer_transfer_counter is not None:
-            self.layer_transfer_counter.wait_until(layer_id - self.start_layer)
-        return self.index_k_with_scale_buffer[layer_id - self.start_layer]
+        return self.index_key_cache.get_local_buffer(layer_id)
 
     def get_index_k_continuous(
         self,
@@ -4437,12 +4407,7 @@ class DSATokenToKVPool(MLATokenToKVPool):
         seq_len: int,
         page_indices: torch.Tensor,
     ):
-        if self.layer_transfer_counter is not None:
-            self.layer_transfer_counter.wait_until(layer_id - self.start_layer)
-        buf = self.index_k_with_scale_buffer[layer_id - self.start_layer]
-        return index_buf_accessor.GetK.execute(
-            self, buf, seq_len=seq_len, page_indices=page_indices
-        )
+        return self.index_key_cache.get_k_continuous(layer_id, seq_len, page_indices)
 
     def get_index_k_scale_continuous(
         self,
@@ -4450,11 +4415,8 @@ class DSATokenToKVPool(MLATokenToKVPool):
         seq_len: int,
         page_indices: torch.Tensor,
     ):
-        if self.layer_transfer_counter is not None:
-            self.layer_transfer_counter.wait_until(layer_id - self.start_layer)
-        buf = self.index_k_with_scale_buffer[layer_id - self.start_layer]
-        return index_buf_accessor.GetS.execute(
-            self, buf, seq_len=seq_len, page_indices=page_indices
+        return self.index_key_cache.get_k_scale_continuous(
+            layer_id, seq_len, page_indices
         )
 
     def get_index_k_scale_buffer(
@@ -4465,27 +4427,8 @@ class DSATokenToKVPool(MLATokenToKVPool):
         seq_len_sum: int,
         max_seq_len: int,
     ):
-        """
-        Fused method to get both index K and scale data in a single call using Triton.
-        More efficient than calling get_index_k_continuous and get_index_k_scale_continuous separately.
-
-        :param layer_id: Layer index
-        :param seq_len: Sequence length
-        :param page_indices: Page indices tensor
-        :return: tuple of (k_fp8, k_scale) where
-                 k_fp8: (seq_len, index_head_dim), uint8
-                 k_scale: (seq_len, 4), uint8
-        """
-        if self.layer_transfer_counter is not None:
-            self.layer_transfer_counter.wait_until(layer_id - self.start_layer)
-        buf = self.index_k_with_scale_buffer[layer_id - self.start_layer]
-        return index_buf_accessor.GetKAndS.execute(
-            self,
-            buf,
-            page_indices=page_indices,
-            seq_len_tensor=seq_len_tensor,
-            seq_len_sum=seq_len_sum,
-            max_seq_len=max_seq_len,
+        return self.index_key_cache.get_k_and_scale(
+            layer_id, seq_len_tensor, page_indices, seq_len_sum, max_seq_len
         )
 
     def set_index_k_scale_buffer(
@@ -4495,68 +4438,20 @@ class DSATokenToKVPool(MLATokenToKVPool):
         index_k: torch.Tensor,
         index_k_scale: torch.Tensor,
     ) -> None:
-        buf = self.index_k_with_scale_buffer[layer_id - self.start_layer]
-        index_buf_accessor.SetKAndS.execute(
-            pool=self, buf=buf, loc=loc, index_k=index_k, index_k_scale=index_k_scale
-        )
+        self.index_key_cache.store_quantized(layer_id, loc, index_k, index_k_scale)
 
     def get_cpu_copy(self, indices, mamba_indices=None):
-        # DSA keeps a page-indexed index_k_with_scale_buffer alongside kv_buffer.
-        # Retract frees the slots/pages and they get reused by other reqs'
-        # set_index_k_scale_buffer, so we must offload it here too -- otherwise
-        # resume restores kv_buffer but leaves foreign index/scale in place and
-        # DSA attention reads garbage at those token positions.
         kv_cache_cpu = super().get_cpu_copy(indices, mamba_indices=mamba_indices)
-
-        page_indices = indices[:: self.page_size] // self.page_size
-        torch.cuda.synchronize()
-        index_k_cpu = []
-        chunk_size = self.cpu_offloading_chunk_size
-        page_chunk_size = max(1, chunk_size // self.page_size)
-        for layer_id in range(self.layer_num):
-            index_k_cpu.append([])
-            for i in range(0, len(page_indices), page_chunk_size):
-                chunk_page_indices = page_indices[i : i + page_chunk_size]
-                idx_cpu = self.index_k_with_scale_buffer[layer_id][
-                    chunk_page_indices
-                ].to("cpu", non_blocking=True)
-                index_k_cpu[-1].append(idx_cpu)
-        torch.cuda.synchronize()
-
-        return {"kv": kv_cache_cpu, "index_k": index_k_cpu}
+        return {"kv": kv_cache_cpu, "index_k": self.index_key_cache.cpu_copy(indices)}
 
     def load_cpu_copy(self, kv_cache_cpu_dict, indices, mamba_indices=None):
         super().load_cpu_copy(
             kv_cache_cpu_dict["kv"], indices, mamba_indices=mamba_indices
         )
-
-        page_indices = indices[:: self.page_size] // self.page_size
-        index_k_cpu = kv_cache_cpu_dict["index_k"]
-        torch.cuda.synchronize()
-        chunk_size = self.cpu_offloading_chunk_size
-        page_chunk_size = max(1, chunk_size // self.page_size)
-        for layer_id in range(self.layer_num):
-            for i in range(0, len(page_indices), page_chunk_size):
-                chunk_page_indices = page_indices[i : i + page_chunk_size]
-                idx_cpu = index_k_cpu[layer_id][i // page_chunk_size]
-                assert idx_cpu.shape[0] == len(chunk_page_indices)
-                idx_chunk = idx_cpu.to(
-                    self.index_k_with_scale_buffer[0].device, non_blocking=True
-                )
-                self.index_k_with_scale_buffer[layer_id][chunk_page_indices] = idx_chunk
-        torch.cuda.synchronize()
+        self.index_key_cache.load_cpu_copy(kv_cache_cpu_dict["index_k"], indices)
 
     def get_state_buf_infos(self):
-        data_ptrs = [
-            self.index_k_with_scale_buffer[i].data_ptr() for i in range(self.layer_num)
-        ]
-        data_lens = [
-            self.index_k_with_scale_buffer[i].nbytes for i in range(self.layer_num)
-        ]
-        item_lens = [
-            self.index_k_with_scale_buffer[i][0].nbytes for i in range(self.layer_num)
-        ]
-        return data_ptrs, data_lens, item_lens
+        return self.index_key_cache.state_buf_infos()
 
     def get_kv_size_bytes(self):
         kv_size_bytes = super().get_kv_size_bytes()
diff --git a/python/sglang/srt/models/deepseek_nextn.py b/python/sglang/srt/models/deepseek_nextn.py
index a722e6076..8a4151383 100644
--- a/python/sglang/srt/models/deepseek_nextn.py
+++ b/python/sglang/srt/models/deepseek_nextn.py
@@ -35,6 +35,7 @@ from sglang.srt.layers.attention.dsa.utils import (
     is_dsa_enable_prefill_cp,
     is_dsa_prefill_cp_round_robin_split,
 )
+from sglang.srt.layers.attention.index_topk_share import IndexTopKShareState
 from sglang.srt.layers.cp.utils import cp_gather_after_forward, is_cp_v2_active
 from sglang.srt.layers.layernorm import RMSNorm
 from sglang.srt.layers.linear import ReplicatedLinear
@@ -261,14 +262,7 @@ class DeepseekModelNextN(nn.Module):
                 hidden_states = cp_split_and_rebuild_data(forward_batch, hidden_states)
                 positions = cp_split_and_rebuild_position(forward_batch, positions)
             residual = None
-            seed_buf = (
-                forward_batch.spec_info.dsa_seed_topk_capture
-                if forward_batch.forward_mode.is_extend(include_draft_extend_v2=True)
-                else None
-            )
-            should_update_dsa_topk_indices = (
-                forward_batch.reuse_dsa_topk_indices or seed_buf is not None
-            )
+            index_topk_share = IndexTopKShareState.from_mtp_carry(forward_batch)
             with get_global_expert_distribution_recorder().disable_this_region():
                 hidden_states, residual, topk_indices = self.decoder(
                     positions,
@@ -276,11 +270,7 @@ class DeepseekModelNextN(nn.Module):
                     forward_batch,
                     residual,
                     zero_allocator,
-                    prev_topk_indices=(
-                        forward_batch.spec_info.dsa_topk_indices
-                        if forward_batch.reuse_dsa_topk_indices
-                        else None
-                    ),
+                    prev_topk_indices=index_topk_share.topk_indices,
                 )
             if not forward_batch.forward_mode.is_idle():
                 if residual is not None:
@@ -296,7 +286,7 @@ class DeepseekModelNextN(nn.Module):
                         forward_batch,
                         torch.cuda.current_stream(),
                     )
-                    if should_update_dsa_topk_indices and topk_indices is not None:
+                    if index_topk_share.should_publish and topk_indices is not None:
                         topk_indices = _gather_dsa_topk_indices_for_cp(
                             topk_indices,
                             local_num_tokens,
@@ -306,21 +296,12 @@ class DeepseekModelNextN(nn.Module):
                         )
                 elif (
                     cp_v2_active
-                    and should_update_dsa_topk_indices
+                    and index_topk_share.should_publish
                     and topk_indices is not None
                 ):
                     topk_indices = cp_gather_after_forward(topk_indices, forward_batch)
-            if should_update_dsa_topk_indices and topk_indices is not None:
-                if forward_batch.reuse_dsa_topk_indices:
-                    forward_batch.spec_info.dsa_topk_indices = topk_indices
-                if seed_buf is not None:
-                    sel = forward_batch.spec_info.dsa_seed_topk_select
-                    src = (
-                        topk_indices[: seed_buf.shape[0]]
-                        if sel is None
-                        else topk_indices[sel]
-                    )
-                    seed_buf[: src.shape[0]].copy_(src)
+            index_topk_share.update(topk_indices)
+            index_topk_share.publish()
         finally:
             exit_stack.close()
 
diff --git a/python/sglang/srt/models/deepseek_v2.py b/python/sglang/srt/models/deepseek_v2.py
index 61a4be735..5c4c4b244 100644
--- a/python/sglang/srt/models/deepseek_v2.py
+++ b/python/sglang/srt/models/deepseek_v2.py
@@ -67,6 +67,7 @@ from sglang.srt.layers.attention.dsa.utils import (
     dsa_use_prefill_cp,
     is_dsa_enable_prefill_cp,
 )
+from sglang.srt.layers.attention.index_topk_share import IndexTopKShareState
 from sglang.srt.layers.aux_hidden_states import (
     AuxHiddenStateAccumulator,
     AuxHiddenStatePacker,
@@ -2729,18 +2730,21 @@ class DeepseekV2Model(nn.Module):
             else:
                 hidden_states = input_embeds
             residual = None
+            initial_topk_indices = None
         else:
             assert pp_proxy_tensors is not None
             hidden_states = pp_proxy_tensors["hidden_states"]
             residual = pp_proxy_tensors["residual"]
-            topk_indices = pp_proxy_tensors.tensors.get("topk_indices")
+            initial_topk_indices = pp_proxy_tensors.tensors.get("topk_indices")
+        index_topk_share = IndexTopKShareState(forward_batch, initial_topk_indices)
+        if not self.pp_group.is_first_rank:
             assert not (
                 not forward_batch.forward_mode.is_idle()
                 and hidden_states.shape[0] != 0
                 and self.use_dsa
                 and dsa_forward_uses_topk
                 and dsa_layer_skips_topk(self.config, self.start_layer)
-                and topk_indices is None
+                and index_topk_share.topk_indices is None
             ), (
                 f"PP stage starting at layer {self.start_layer} requires DSA "
                 "topk_indices from the previous stage."
@@ -2802,8 +2806,6 @@ class DeepseekV2Model(nn.Module):
                 normal_end_layer = normal_start_layer = 0
         # Append-compatible, so the shared capture path below is unchanged.
         aux_hidden_states = AuxHiddenStatePacker(len(self.layers_to_capture))
-        if self.pp_group.is_first_rank:
-            topk_indices = None
         for i in range(normal_start_layer, normal_end_layer):
             # NOTE: torch dynamo does not support graph break in context manager
             ctx = (
@@ -2821,7 +2823,7 @@ class DeepseekV2Model(nn.Module):
                     zero_allocator,
                     gemm_output_zero_allocator,
                     llama_4_scaling,
-                    prev_topk_indices=topk_indices,
+                    prev_topk_indices=index_topk_share.topk_indices,
                     captured_last_layer_outputs=(
                         aux_hidden_states if i in self.layers_to_capture else None
                     ),
@@ -2829,6 +2831,7 @@ class DeepseekV2Model(nn.Module):
                         i
                     ),
                 )
+                index_topk_share.update(topk_indices)
 
         if normal_end_layer != self.end_layer:
             hidden_states, residual = model_forward_maybe_tbo(
@@ -2855,6 +2858,7 @@ class DeepseekV2Model(nn.Module):
                 and self.end_layer < self.config.num_hidden_layers
                 and dsa_layer_skips_topk(self.config, self.end_layer)
             ):
+                topk_indices = index_topk_share.topk_indices
                 if (
                     not forward_batch.forward_mode.is_idle()
                     and hidden_states.shape[0] != 0
diff --git a/python/sglang/srt/speculative/eagle_worker_v2.py b/python/sglang/srt/speculative/eagle_worker_v2.py
index 8c385b353..682c60937 100644
--- a/python/sglang/srt/speculative/eagle_worker_v2.py
+++ b/python/sglang/srt/speculative/eagle_worker_v2.py
@@ -18,6 +18,7 @@ from sglang.srt.hardware_backend.npu.graph_runner.eagle_draft_npu_graph_runner i
 from sglang.srt.hardware_backend.npu.graph_runner.npu_graph_runner import NPUGraphRunner
 from sglang.srt.kv_canary.runner.canary_manager import context_tuple
 from sglang.srt.layers.attention.flashinfer_backend import FlashInferAttnBackend
+from sglang.srt.layers.attention.index_topk_share import IndexTopKShareState
 from sglang.srt.layers.attention.tokenspeed_mla_backend import TokenspeedMLABackend
 from sglang.srt.layers.attention.triton_backend import TritonAttnBackend
 from sglang.srt.layers.attention.trtllm_mha_backend import TRTLLMHAAttnBackend
@@ -599,103 +600,96 @@ class EagleDraftWorker(EagleDraftWorkerBase):
 
         # Forward multiple steps
         scores = None
-        if self.index_share_for_mtp_iteration:
-            forward_batch.reuse_dsa_topk_indices = True
-            # Keep the draft-extend seed so step 0 reuses it; else recompute it.
-            if not (
-                self.seed_dsa_topk_from_draft_extend
-                and spec_info.dsa_topk_indices is not None
-            ):
-                spec_info.dsa_topk_indices = None
-        for i in range(self.speculative_num_steps):
-            if draft_tokens_topk1 is not None:
-                input_ids = topk_index.flatten()
-            else:
-                input_ids, hidden_states, scores, tree_info = select_top_k_tokens(
-                    i, topk_p, topk_index, hidden_states, scores, self.topk
-                )
-                score_list.append(tree_info[0])
-                token_list.append(tree_info[1])
-                parents_list.append(tree_info[2])
-
-            # We don't need to run the last forward. we get 1 token from draft prefill and (#spec steps - 1) tokens here
-            if i == self.speculative_num_steps - 1:
-                break
-
-            # Set inputs
-            forward_batch.input_ids = input_ids
-            # Qwen3-MoE MTP uses a fused RoPE + KV-store path whose cache_loc
-            # argument must be contiguous.
-            if (
-                self.draft_runner.model_config.hf_config.architectures[0]
-                == "Qwen3MoeForCausalLMMTP"
-            ):
-                out_cache_loc = out_cache_loc.contiguous()
-            forward_batch.out_cache_loc = out_cache_loc[i]
-            spec_info.hidden_states = hidden_states
-
-            # Run forward under a per-step ForwardContext so the model layer
-            # reads attn_backends[i] for the i-th draft step, plus a canary
-            # index context so canary tracks which draft step is active.
-            canary_index_ctx = (
-                c.with_active_single_forward_manager(i)
-                if (c := self.draft_runner.canary_manager) is not None
-                else contextlib.nullcontext()
-            )
-            with (
-                forward_context(
-                    ForwardContext(
-                        attn_backend=self.draft_attn_backend.attn_backends[i]
-                    )
-                ),
-                canary_index_ctx,
-            ):
-                logits_output = self.draft_runner.forward(forward_batch).logits_output
-            maybe_detect_nan(logits_output.next_token_logits, f"draft_forward step {i}")
-            maybe_detect_inf(logits_output.next_token_logits, f"draft_forward step {i}")
-            if get_spec().speculative_use_rejection_sampling:
-                probs, topk_p, topk_index = sample_draft_proposal(
-                    logits_output.next_token_logits,
-                    forward_batch.sampling_info.temperatures,
-                )
-                draft_probs_list.append(probs)
-                forward_batch.positions.add_(1)
-            elif self.topk == 1 and not _is_hip:
-                if _is_cuda:
-                    # The positions advance is fused into the kernel.
-                    topk_p, topk_index = draft_topk1_postprocess(
-                        logits_output.next_token_logits,
-                        forward_batch.positions,
-                        draft_tokens_topk1,
-                        i + 1,
-                    )
+        with IndexTopKShareState.mtp_iteration(
+            forward_batch,
+            enabled=self.index_share_for_mtp_iteration,
+            keep_carry_seed=self.seed_dsa_topk_from_draft_extend,
+        ):
+            for i in range(self.speculative_num_steps):
+                if draft_tokens_topk1 is not None:
+                    input_ids = topk_index.flatten()
                 else:
-                    topk_index = torch.argmax(
-                        logits_output.next_token_logits, dim=-1, keepdim=True
+                    input_ids, hidden_states, scores, tree_info = select_top_k_tokens(
+                        i, topk_p, topk_index, hidden_states, scores, self.topk
                     )
-                    topk_p = torch.ones_like(topk_index, dtype=torch.float32)
-                    forward_batch.positions.add_(1)
-            else:
-                probs = renorm_draft_probs(
-                    logits_output.next_token_logits,
-                    forward_batch.sampling_info,
-                    get_spec().speculative_use_rejection_sampling,
-                )
-                topk_p, topk_index = fast_topk(probs, self.topk, dim=-1)
-                forward_batch.positions.add_(1)
-            maybe_detect_oob(
-                topk_index,
-                0,
-                logits_output.next_token_logits.shape[-1],
-                f"draft_forward step {i}: topk_index OOB vs vocab_size={logits_output.next_token_logits.shape[-1]}",
-            )
-            if self.hot_token_id is not None:
-                topk_index = self.hot_token_id[topk_index]
-            hidden_states = logits_output.hidden_states
+                    score_list.append(tree_info[0])
+                    token_list.append(tree_info[1])
+                    parents_list.append(tree_info[2])
 
-        if self.index_share_for_mtp_iteration:
-            spec_info.dsa_topk_indices = None
-            forward_batch.reuse_dsa_topk_indices = False
+                if i == self.speculative_num_steps - 1:
+                    break
+
+                forward_batch.input_ids = input_ids
+                # Qwen3-MoE MTP uses a fused RoPE + KV-store path whose cache_loc
+                # argument must be contiguous.
+                if (
+                    self.draft_runner.model_config.hf_config.architectures[0]
+                    == "Qwen3MoeForCausalLMMTP"
+                ):
+                    out_cache_loc = out_cache_loc.contiguous()
+                forward_batch.out_cache_loc = out_cache_loc[i]
+                spec_info.hidden_states = hidden_states
+
+                canary_index_ctx = (
+                    c.with_active_single_forward_manager(i)
+                    if (c := self.draft_runner.canary_manager) is not None
+                    else contextlib.nullcontext()
+                )
+                with (
+                    forward_context(
+                        ForwardContext(
+                            attn_backend=self.draft_attn_backend.attn_backends[i]
+                        )
+                    ),
+                    canary_index_ctx,
+                ):
+                    logits_output = self.draft_runner.forward(
+                        forward_batch
+                    ).logits_output
+                maybe_detect_nan(
+                    logits_output.next_token_logits, f"draft_forward step {i}"
+                )
+                maybe_detect_inf(
+                    logits_output.next_token_logits, f"draft_forward step {i}"
+                )
+                if get_spec().speculative_use_rejection_sampling:
+                    probs, topk_p, topk_index = sample_draft_proposal(
+                        logits_output.next_token_logits,
+                        forward_batch.sampling_info.temperatures,
+                    )
+                    draft_probs_list.append(probs)
+                    forward_batch.positions.add_(1)
+                elif self.topk == 1 and not _is_hip:
+                    if _is_cuda:
+                        topk_p, topk_index = draft_topk1_postprocess(
+                            logits_output.next_token_logits,
+                            forward_batch.positions,
+                            draft_tokens_topk1,
+                            i + 1,
+                        )
+                    else:
+                        topk_index = torch.argmax(
+                            logits_output.next_token_logits, dim=-1, keepdim=True
+                        )
+                        topk_p = torch.ones_like(topk_index, dtype=torch.float32)
+                        forward_batch.positions.add_(1)
+                else:
+                    probs = renorm_draft_probs(
+                        logits_output.next_token_logits,
+                        forward_batch.sampling_info,
+                        get_spec().speculative_use_rejection_sampling,
+                    )
+                    topk_p, topk_index = fast_topk(probs, self.topk, dim=-1)
+                    forward_batch.positions.add_(1)
+                maybe_detect_oob(
+                    topk_index,
+                    0,
+                    logits_output.next_token_logits.shape[-1],
+                    f"draft_forward step {i}: topk_index OOB vs vocab_size={logits_output.next_token_logits.shape[-1]}",
+                )
+                if self.hot_token_id is not None:
+                    topk_index = self.hot_token_id[topk_index]
+                hidden_states = logits_output.hidden_states
 
         draft_probs = (
             torch.stack(draft_probs_list, dim=1)
diff --git a/test/registered/unit/layers/attention/test_index_topk_share.py b/test/registered/unit/layers/attention/test_index_topk_share.py
new file mode 100644
index 000000000..9332a3520
--- /dev/null
+++ b/test/registered/unit/layers/attention/test_index_topk_share.py
@@ -0,0 +1,163 @@
+import sys
+from types import SimpleNamespace
+
+import pytest
+import torch
+
+from sglang.srt.layers.attention.index_topk_share import IndexTopKShareState
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=2, suite="base-a-test-cpu")
+
+
+def _batch(
+    reuse: bool, carried, *, is_extend: bool = False, seed_buf=None, seed_select=None
+) -> SimpleNamespace:
+    return SimpleNamespace(
+        reuse_dsa_topk_indices=reuse,
+        forward_mode=SimpleNamespace(
+            is_extend=lambda include_draft_extend_v2: is_extend
+        ),
+        spec_info=SimpleNamespace(
+            dsa_topk_indices=carried,
+            dsa_seed_topk_capture=seed_buf,
+            dsa_seed_topk_select=seed_select,
+        ),
+    )
+
+
+def test_mtp_carry_is_empty_when_reuse_disabled():
+    batch = _batch(reuse=False, carried="old")
+    state = IndexTopKShareState.from_mtp_carry(batch)
+
+    assert state.topk_indices is None
+    state.update("new")
+    state.publish()
+
+    assert state.topk_indices == "new"
+    assert batch.spec_info.dsa_topk_indices == "old"
+
+
+def test_mtp_carry_reads_and_publishes_when_reuse_enabled():
+    batch = _batch(reuse=True, carried="old")
+    state = IndexTopKShareState.from_mtp_carry(batch)
+
+    assert state.topk_indices == "old"
+    state.update("new")
+    state.publish()
+
+    assert state.topk_indices == "new"
+    assert batch.spec_info.dsa_topk_indices == "new"
+
+
+def test_target_carry_stays_local_without_publish():
+    batch = _batch(reuse=False, carried="batch")
+    state = IndexTopKShareState(batch, "layer")
+
+    assert state.topk_indices == "layer"
+    assert batch.spec_info.dsa_topk_indices == "batch"
+
+    state.update(None)
+
+    assert state.topk_indices is None
+    assert batch.spec_info.dsa_topk_indices == "batch"
+
+
+def test_target_none_does_not_fall_back_to_mtp_carry():
+    batch = _batch(reuse=True, carried="batch")
+    state = IndexTopKShareState(batch, None)
+
+    assert state.topk_indices is None
+
+    state.update("layer")
+
+    assert state.topk_indices == "layer"
+    assert batch.spec_info.dsa_topk_indices == "batch"
+
+
+def test_publish_captures_draft_extend_seed():
+    seed_buf = torch.zeros(2, 3, dtype=torch.int64)
+    batch = _batch(reuse=False, carried=None, is_extend=True, seed_buf=seed_buf)
+    state = IndexTopKShareState.from_mtp_carry(batch)
+
+    assert state.should_publish
+    state.update(torch.arange(12, dtype=torch.int64).view(4, 3))
+    state.publish()
+
+    assert torch.equal(seed_buf, torch.arange(6, dtype=torch.int64).view(2, 3))
+    assert batch.spec_info.dsa_topk_indices is None
+
+
+def test_seed_buffer_is_ignored_outside_extend():
+    seed_buf = torch.zeros(2, 3, dtype=torch.int64)
+    batch = _batch(reuse=False, carried=None, is_extend=False, seed_buf=seed_buf)
+    state = IndexTopKShareState.from_mtp_carry(batch)
+
+    assert not state.should_publish
+    state.update(torch.ones(4, 3, dtype=torch.int64))
+    state.publish()
+
+    assert torch.equal(seed_buf, torch.zeros(2, 3, dtype=torch.int64))
+
+
+def test_mtp_iteration_clears_batch_state():
+    batch = _batch(reuse=False, carried="stale")
+
+    with IndexTopKShareState.mtp_iteration(batch) as state:
+        assert state is not None
+        assert batch.reuse_dsa_topk_indices
+        assert batch.spec_info.dsa_topk_indices is None
+        batch.spec_info.dsa_topk_indices = "draft-topk"
+
+    assert not batch.reuse_dsa_topk_indices
+    assert batch.spec_info.dsa_topk_indices is None
+
+
+def test_mtp_iteration_clears_batch_state_on_exception():
+    batch = _batch(reuse=False, carried=None)
+
+    with pytest.raises(RuntimeError, match="draft step blew up"):
+        with IndexTopKShareState.mtp_iteration(batch):
+            batch.spec_info.dsa_topk_indices = "draft-topk"
+            raise RuntimeError("draft step blew up")
+
+    assert not batch.reuse_dsa_topk_indices
+    assert batch.spec_info.dsa_topk_indices is None
+
+
+def test_disabled_mtp_iteration_is_passthrough():
+    batch = _batch(reuse=False, carried="untouched")
+
+    with IndexTopKShareState.mtp_iteration(batch, enabled=False) as state:
+        assert state is None
+        assert not batch.reuse_dsa_topk_indices
+        assert batch.spec_info.dsa_topk_indices == "untouched"
+
+    assert not batch.reuse_dsa_topk_indices
+    assert batch.spec_info.dsa_topk_indices == "untouched"
+
+
+def test_mtp_iteration_preserves_draft_extend_seed():
+    batch = _batch(reuse=False, carried="extend-seed")
+
+    with IndexTopKShareState.mtp_iteration(batch, keep_carry_seed=True) as state:
+        assert state is not None
+        assert batch.spec_info.dsa_topk_indices == "extend-seed"
+        assert state.topk_indices == "extend-seed"
+
+    assert not batch.reuse_dsa_topk_indices
+    assert batch.spec_info.dsa_topk_indices is None
+
+
+def test_mtp_iteration_clears_missing_draft_extend_seed():
+    batch = _batch(reuse=False, carried=None)
+
+    with IndexTopKShareState.mtp_iteration(batch, keep_carry_seed=True):
+        assert batch.spec_info.dsa_topk_indices is None
+
+    assert not batch.reuse_dsa_topk_indices
+    assert batch.spec_info.dsa_topk_indices is None
+
+
+if __name__ == "__main__":
+    sys.exit(pytest.main([__file__]))
diff --git a/test/registered/unit/mem_cache/test_dsa_layer_split_broadcast.py b/test/registered/unit/mem_cache/test_dsa_layer_split_broadcast.py
index 0e2c1c7cc..ad325bd97 100644
--- a/test/registered/unit/mem_cache/test_dsa_layer_split_broadcast.py
+++ b/test/registered/unit/mem_cache/test_dsa_layer_split_broadcast.py
@@ -104,8 +104,12 @@ def _run(rank: int, world: int, port: int):
     # Indexer buffer owner-broadcast: owner writes a layer-distinct value, then
     # every rank must read it back for every layer.
     for layer_id in range(LAYER_NUM):
+        store_buf = pool.get_index_k_with_scale_buffer(layer_id)
+        assert (
+            store_buf.data_ptr() == pool.index_k_with_scale_buffer[layer_id].data_ptr()
+        )
         if pool._is_layer_owned(layer_id):
-            pool.index_k_with_scale_buffer[layer_id].fill_(layer_id + 10)
+            store_buf.fill_(layer_id + 10)
     torch.cuda.synchronize()
     torch.distributed.barrier()
     for layer_id in range(LAYER_NUM):