[AMD] Restore SWA reprefill-tail on UnifiedRadixCache when HiCache is off (#32759)

Co-authored-by: Thomas Wang <thomawan@amd.com>

python/sglang/srt/mem_cache/unified_radix_cache.py

@@ -3071,20 +3071,28 @@ class UnifiedRadixCache(BasePrefixCache):
 
     def swa_reprefill_tail_tokens(self) -> int:
         """
-        Only unified_kv + HiCache needs this: SWA lives in a per-request ring
-        (state_slot/pos), not content-stable and never offloaded to host, so a
+        Only unified_kv needs this: SWA lives in a per-request ring
+        (state_slot/pos), not content-stable and never stored in the tree, so a
         reused prefix's trailing sliding window would read another request's
-        stale ring slots. Re-prefilling that window rewrites this request's ring
-        (what plain radix reuse does via its SWA match gate). 0 for every other
-        layout.
+        stale ring slots. Re-prefilling that window rewrites this request's ring.
+
+        Applies to plain radix reuse as well as HiCache -- the ring is stale
+        either way. Returns 0 once SWA has a host pool to restore exact contents
+        from, and for every non-unified_kv layout, whose SWA slots are
+        content-stable.
         """
-        swa = self.components.get(ComponentType.SWA)
-        unified_compress_only_hicache = (
-            self.cache_controller is not None
-            and swa is not None
-            and not self.tree_core.has_swa_host_pool
+        from sglang.kernels.ops.attention.dsv4.unified_kv_kernels.env_gate import (
+            is_unified_kv_triton,
         )
-        return swa.sliding_window_size if unified_compress_only_hicache else 0
+
+        swa = self.components.get(ComponentType.SWA)
+        if swa is None or not swa.sliding_window_size:
+            return 0
+        if not is_unified_kv_triton():
+            return 0
+        if self.tree_core.has_swa_host_pool:
+            return 0
+        return swa.sliding_window_size
 
     def swa_retain_floor(self, req) -> int | None:
         if not self.is_mamba_enabled or self._sliding_window_size is None: