Fix unnecessary gather/scatter on CPU for non-contiguous Mamba statepool (#31754)

python/sglang/srt/layers/attention/linear/gdn_backend.py

@@ -488,11 +488,16 @@ class GDNAttnBackend(MambaAttnBackendBase):
         # slot layout, so they silently drop the write to the strided envelope
         # pool. Run them on contiguous per-sequence copies (identity-indexed) and
         # scatter the result back. No-op for the default contiguous pool.
+        # CPU kernels (causal_conv1d_fwd_cpu, chunk_gated_delta_rule_cpu) use
+        # proper indexed writes and handle non-contiguous pools directly via
+        # cache_indices, so the gather/scatter round-trip is unnecessary on CPU.
         # TODO(ch-wan): drop these .contiguous() copies by making the prefill conv
         # and chunk_gated_delta_rule kernels honor the pool's real slot stride +
         # int64 indexing, like packed_decode / causal_conv1d_update already do.
-        needs_state_gather = (not is_target_verify) and (
-            not conv_states.is_contiguous() or not ssm_states.is_contiguous()
+        needs_state_gather = (
+            (not is_target_verify)
+            and (not is_cpu())
+            and (not conv_states.is_contiguous() or not ssm_states.is_contiguous())
         )
         if needs_state_gather:
             conv_states_contig = conv_states[cache_indices].contiguous()