[Bugfix] Disable TRTLLM attention when KV transfer is enabled (#33192)

Signed-off-by: Zhanqiu Hu <zh338@cornell.edu>

vllm/v1/attention/backends/flashinfer.py

@@ -573,6 +573,20 @@ class FlashInferMetadataBuilder(AttentionMetadataBuilder[FlashInferMetadata]):
         # try to use fp8 q if kv cache is fp8, and will fall back to model dtype
         # if TRTLLM attention kernel is not used when building attn metadata
         can_use_trtllm = can_use_trtllm_attention(self.num_qo_heads, self.num_kv_heads)
+
+        # TRTLLM attention requires strictly contiguous KV cache tensors.
+        # When KV transfer (P/D disaggregation) is enabled, the KV cache may be
+        # permuted into non-contiguous views, which causes assertion failures.
+        self._kv_transfer_enabled = vllm_config.kv_transfer_config is not None
+        if can_use_trtllm and self._kv_transfer_enabled:
+            logger.info_once(
+                "TRTLLM attention is disabled because KV transfer "
+                "(P/D disaggregation) is enabled. TRTLLM attention requires "
+                "strictly contiguous KV cache tensors which may not be "
+                "guaranteed with KV transfer."
+            )
+            can_use_trtllm = False
+
         if (
             can_use_trtllm
             and not vllm_config.attention_config.disable_flashinfer_q_quantization
@@ -822,6 +836,9 @@ class FlashInferMetadataBuilder(AttentionMetadataBuilder[FlashInferMetadata]):
             has_sinks=self.has_sinks,
             has_spec=uses_spec_reorder,
         )
+        # KV transfer requires non-contiguous KV cache views, incompatible with TRTLLM
+        if self._kv_transfer_enabled:
+            prefill_use_trtllm = False
         decode_use_trtllm = (
             self.use_trtllm_decode_attention and self.dcp_world_size <= 1
         )