[V1][BugFix] Raise error when selected attn backend is not supported

vllm/attention/selector.py

@@ -144,6 +144,20 @@ def _cached_get_attn_backend(
         if backend_by_env_var is not None:
             selected_backend = backend_name_to_enum(backend_by_env_var)
 
+    if selected_backend is not None and use_v1:
+        if (selected_backend in (_Backend.FLASH_ATTN, _Backend.FLASHINFER,
+                                 _Backend.XFORMERS)):
+            raise ValueError(
+                f"{selected_backend.name} is not compatible with vLLM V1. "
+                "Please either do `export VLLM_ATTENTION_BACKEND="
+                f"{_Backend.FLASH_ATTN_VLLM_V1.name}` or unset it to use "
+                "the default backend.")
+        elif selected_backend not in _Backend.get_v1_backends():
+            raise ValueError(
+                f"{selected_backend.name} attention backend is not compatible "
+                "with vLLM V1. Please use a different backend or unset the "
+                "VLLM_ATTENTION_BACKEND env variable.")
+
     # get device-specific attn_backend
     attention_cls = current_platform.get_attn_backend_cls(
         selected_backend, head_size, dtype, kv_cache_dtype, block_size, use_v1,

vllm/platforms/interface.py

@@ -42,6 +42,10 @@ class _Backend(enum.Enum):
     BLOCK_SPARSE_FLASH_ATTN = enum.auto()
     NO_ATTENTION = enum.auto()
 
+    @classmethod
+    def get_v1_backends(cls) -> Tuple["_Backend", ...]:
+        return (cls.FLASH_ATTN_VLLM_V1, cls.ROCM_FLASH, cls.PALLAS_VLLM_V1)
+
 
 class PlatformEnum(enum.Enum):
     CUDA = enum.auto()