[Bugfix] Restrict FlashInfer cuDNN FP8 ViT attention gate to Blackwell (SM 100) (vllm-project#45251)

Signed-off-by: Wentian Byte <3400259131@qq.com>

Author: wentian-byte

vllm/model_executor/layers/attention/mm_encoder_attention.py

@@ -396,8 +396,9 @@ def _init_fp8_state(self) -> None:
         if not is_flashinfer_cudnn_fp8_prefill_attn_supported():
             raise ValueError(
                 "mm_encoder_attn_dtype='fp8' requires the FlashInfer "
-                "cuDNN backend with cuDNN >= 9.17.1 on a GPU with native "
-                "FP8 support."
+                "cuDNN backend with cuDNN >= 9.17.1 on Blackwell (SM 100) "
+                "or newer. cuDNN's FP8 SDPA path with bf16/fp16 output is "
+                "not available on Hopper (H100/H200) or earlier."
             )
 
         self.fp8_enabled = True

vllm/utils/flashinfer.py

@@ -934,20 +934,27 @@ def should_use_flashinfer_for_blockscale_fp8_gemm(
     return should_use_flashinfer
 
 
-_MIN_CUDNN_FP8 = 91701  # cuDNN >= 9.17.1 required for FP8 attention
+_MIN_CUDNN_FP8 = 91701  # cuDNN >= 9.17.1 required for FP8 ViT attention
 
 
 @functools.cache
 def is_flashinfer_cudnn_fp8_prefill_attn_supported() -> bool:
     """Check if FP8 ViT attention is supported on this platform.
 
-    Requires native FP8 hardware support, the FlashInfer cuDNN backend,
+    Requires Blackwell (SM 100) or newer, the FlashInfer cuDNN backend,
     and cuDNN >= 9.17.1.
+
+    cuDNN's FP8 SDPA forward path with bf16/fp16 output (used by
+    ``MMEncoderAttention._forward_flashinfer``) gates internally on
+    ``prop.major >= 10``; on Hopper it raises a misleading
+    ``cudnnGraphNotSupportedError: ... cuDNN version 9.13.0 and newer``
+    even when the installed cuDNN is new enough. See PR #38065 for the
+    original Blackwell-only design intent.
     """
     from vllm.v1.attention.backends.registry import AttentionBackendEnum
 
-    # cuDNN SDPA FP8 requires Hopper (SM 90) or newer.
-    if not current_platform.has_device_capability(90):
+    # cuDNN SDPA FP8 with bf16/fp16 output requires Blackwell (SM 100) or newer.
+    if not current_platform.has_device_capability(100):
         return False
 
     try: