[Bugfix][Spec Decode] Scope DSpark backend inheritance to DeepSeek V4

vllm/v1/worker/gpu/spec_decode/dspark/utils.py

@@ -3,13 +3,32 @@
 
 import torch.nn as nn
 
-from vllm.config import VllmConfig, replace
+from vllm.config import ModelConfig, VllmConfig, replace
 from vllm.distributed.parallel_state import get_pp_group
-from vllm.model_executor.model_loader import get_model
-from vllm.v1.worker.gpu.spec_decode.eagle.utils import (
-    _should_share,
-    get_target_lm_head,
-)
+from vllm.logger import init_logger
+from vllm.v1.attention.backends.registry import AttentionBackendEnum
+
+logger = init_logger(__name__)
+
+
+def _resolve_dspark_attention_backend(
+    draft_model_config: ModelConfig,
+    draft_backend: AttentionBackendEnum | None,
+    target_backend: AttentionBackendEnum | None,
+) -> AttentionBackendEnum | None:
+    if draft_backend is not None:
+        return draft_backend
+    # DeepSeek-V4 draft layers share the target's KV-cache layout. Other
+    # DSpark architectures may use a different attention kind.
+    if draft_model_config.hf_config.model_type == "deepseek_v4":
+        if target_backend is not None:
+            logger.info_once(
+                "Using the target model's %s attention backend for the "
+                "DeepSeek-V4 DSpark drafter.",
+                target_backend.name,
+            )
+        return target_backend
+    return None
 
 
 def load_dspark_model(target_model: nn.Module, vllm_config: VllmConfig) -> nn.Module:
@@ -18,13 +37,18 @@ def load_dspark_model(target_model: nn.Module, vllm_config: VllmConfig) -> nn.Mo
     draft_model_config = speculative_config.draft_model_config
 
     from vllm.compilation.backends import set_model_tag
+    from vllm.model_executor.model_loader import get_model
     from vllm.model_executor.models.qwen3_dflash import dflash_has_any_non_causal
     from vllm.model_executor.models.utils import get_draft_quant_config
+    from vllm.v1.worker.gpu.spec_decode.eagle.utils import (
+        _should_share,
+        get_target_lm_head,
+    )
 
-    # None re-runs backend auto-selection for the draft, which can pick a
-    # different attention class than the target; fall back to the target's.
-    draft_attention_backend = (
-        speculative_config.attention_backend or vllm_config.attention_config.backend
+    draft_attention_backend = _resolve_dspark_attention_backend(
+        draft_model_config,
+        speculative_config.attention_backend,
+        vllm_config.attention_config.backend,
     )
 
     draft_vllm_config = replace(