v1: Add Whisper model support (encoder-decoder)

This brings Whisper support to V1 to close one of the remaining
feature gaps with V0. Most of the changes apply to encoder-decoder
models generally, though Whisper is the only one explicitly tested
and is the only encoder-decoder model updated to support V1.

**Whisper Model Implementation:**
- Remove SupportsV0Only interface constraint to enable V1 compatibility
- Update get_multimodal_embeddings() to return list format required by V1

**Flash Attention Backend:**
- Add encoder attention metadata fields (encoder_seq_start_loc, max_encoder_seq_len, cross_slot_mapping)
- Implement encoder self-attention support without using KV cache
- Add cross-attention support for encoder-decoder models with proper KV cache handling

**KV Cache Manager:**
- Introduce CrossAttentionManager for handling cross-attention KV cache in encoder-decoder models
- Add CrossAttentionSpec for cross-attention cache specification with encoder-based sizing
- Implement allocate_slots_for_cross_attn() for static encoder-length-based allocation
- Add cross-attention block allocation logic separate from decoder token growth

**Scheduler:**
- Disable prefix caching for encoder-decoder models
- Implement cross-attention block allocation during request scheduling
- Add cross-attention block tracking in state management

**GPU Model Runner:**
- Add encoder input extraction for audio features processing
- Implement encoder attention metadata building for both self-attention and cross-attention
- Add cross-attention KV cache group handling with proper slot mapping
- Modify input batch creation to accommodate encoder sequence lengths
- Add encoder input processing in forward pass with proper device/dtype handling
- Update profiling and memory management for encoder-decoder models

The implementation maintains backward compatibility while adding comprehensive
encoder-decoder support, with particular focus on Whisper's audio processing
pipeline and cross-attention mechanisms between encoder and decoder.

Related to:
- V0 deprecation: #18571
- 2025 Q3 roadmap: #20336

Signed-off-by: Russell Bryant <rbryant@redhat.com>

Author: russellb

vllm/attention/__init__.py

@@ -14,7 +14,6 @@
     "AttentionMetadata",
     "AttentionType",
     "AttentionMetadataBuilder",
-    "Attention",
     "AttentionState",
     "get_attn_backend",
 ]

vllm/attention/backends/flash_attn.py

@@ -1004,6 +1004,4 @@ def _get_causal_option(attn_type: str) -> bool:
         attention (i.e., not encoder, encoder-only, or encoder-decoder), 
         otherwise returns `False`.
     """
-    return not (attn_type == AttentionType.ENCODER
-                or attn_type == AttentionType.ENCODER_ONLY
-                or attn_type == AttentionType.ENCODER_DECODER)
+    return attn_type == AttentionType.DECODER

vllm/inputs/preprocess.py

@@ -869,9 +869,6 @@ def preprocess(
     ) -> ProcessorInputs:
         """Preprocess the input prompt."""
         if self.model_config.is_encoder_decoder:
-            assert not return_mm_hashes, (
-                "Multimodal hashes for encoder-decoder models should not be ",
-                "returned until they are supported on vLLM V1.")
             # Encoder-decoder model requires special mapping of
             # input prompts to encoder & decoder
             return self._process_encoder_decoder_prompt(
@@ -903,9 +900,6 @@ async def preprocess_async(
         [`preprocess`][vllm.inputs.preprocess.InputPreprocessor.preprocess].
         """
         if self.model_config.is_encoder_decoder:
-            assert not return_mm_hashes, (
-                "Multimodal hashes for encoder-decoder models should not be ",
-                "returned until they are supported on vLLM V1.")
             # Encoder-decoder model requires special mapping of
             # input prompts to encoder & decoder
             return await self._process_encoder_decoder_prompt_async(prompt)

vllm/model_executor/models/whisper.py

@@ -42,7 +42,7 @@
 from vllm.transformers_utils.processor import cached_get_processor
 
 from .interfaces import (MultiModalEmbeddings, SupportsMultiModal,
-                         SupportsTranscription, SupportsV0Only)
+                         SupportsTranscription)
 from .utils import (AutoWeightsLoader, WeightsMapper, cast_overflow_tensors,
                     make_layers)
 
@@ -790,7 +790,7 @@ def _get_prompt_updates(
                                         info=WhisperProcessingInfo,
                                         dummy_inputs=WhisperDummyInputsBuilder)
 class WhisperForConditionalGeneration(nn.Module, SupportsTranscription,
-                                      SupportsMultiModal, SupportsV0Only):
+                                      SupportsMultiModal):
     packed_modules_mapping = {
         "self_attn.qkv_proj": [
             "self_attn.q_proj",
@@ -916,10 +916,9 @@ def get_language_model(self) -> torch.nn.Module:
 
     def get_multimodal_embeddings(self,
                                   **kwargs: object) -> MultiModalEmbeddings:
-        # TODO: This method does not obey the interface for SupportsMultiModal.
-        # Refactor this once encoder/decoder support is implemented in V1.
+        # Required as part of SupportsMultiModal interface.
         audio_input = self._parse_and_validate_audio_input(**kwargs)
-        return self.model.get_encoder_outputs(audio_input["input_features"])
+        return [self.model.get_encoder_outputs(audio_input["input_features"])]
 
     def get_input_embeddings(
         self,

vllm/v1/attention/backends/flash_attn.py

@@ -130,6 +130,24 @@ class FlashAttentionMetadata:
     prefix_scheduler_metadata: Optional[torch.Tensor] = None
     max_num_splits: int = 0
 
+    # Begin encoder attn & enc/dec cross-attn fields...
+
+    # (batch_size + 1,). The cumulative sequence lengths of the encoder
+    # sequences in the batch, used to index into sequence. E.g., if the sequence
+    # length is [4, 6], it is [0, 4, 10].
+    encoder_seq_start_loc: Optional[torch.Tensor] = None
+    # Maximum sequence length among encoder sequences
+    max_encoder_seq_len: Optional[int] = None
+    cross_slot_mapping: Optional[torch.Tensor] = None
+
+    @property
+    def is_all_encoder_attn_metadata_set(self) -> bool:
+        """
+        All attention metadata required for encoder attention is set.
+        """
+        return (self.encoder_seq_start_loc is not None
+                and self.max_encoder_seq_len is not None)
+
 
 def _get_sliding_window_configs(
         vllm_config: VllmConfig) -> set[Optional[tuple[int, int]]]:
@@ -207,7 +225,13 @@ def build(self,
         num_reqs = common_attn_metadata.num_reqs
         num_actual_tokens = common_attn_metadata.num_actual_tokens
         max_query_len = common_attn_metadata.max_query_len
-        max_seq_len = int(common_attn_metadata.seq_lens_cpu.max())
+
+        if (common_attn_metadata.cross_slot_mapping is not None
+                and common_attn_metadata.max_encoder_seq_len is not None):
+            # ENCODER_DECODER cross-attention
+            max_seq_len = common_attn_metadata.max_encoder_seq_len
+        else:
+            max_seq_len = int(common_attn_metadata.seq_lens_cpu.max())
         query_start_loc = common_attn_metadata.query_start_loc
         seq_lens = common_attn_metadata.seq_lens
         seq_lens_cpu = common_attn_metadata.seq_lens_cpu
@@ -326,6 +350,10 @@ def schedule(batch_size, cu_query_lens, max_query_len, seqlens,
             suffix_kv_lens=suffix_kv_lens,
             prefix_scheduler_metadata=prefix_scheduler_metadata,
             max_num_splits=max_num_splits,
+            # Encoder/cross-attention fields
+            encoder_seq_start_loc=common_attn_metadata.encoder_seq_start_loc,
+            max_encoder_seq_len=common_attn_metadata.max_encoder_seq_len,
+            cross_slot_mapping=common_attn_metadata.cross_slot_mapping,
         )
         return attn_metadata
 
@@ -380,18 +408,32 @@ def __init__(
 
         FlashAttentionBackend.validate_head_size(head_size)
 
-        if attn_type != AttentionType.DECODER:
-            raise NotImplementedError("Encoder self-attention and "
-                                      "encoder/decoder cross-attention "
-                                      "are not implemented for "
-                                      "FlashAttentionImpl")
+        self.attn_type = attn_type
         self.use_irope = use_irope
         self.vllm_flash_attn_version = get_flash_attn_version()
         if is_quantized_kv_cache(self.kv_cache_dtype) \
             and not flash_attn_supports_fp8():
             raise NotImplementedError(
                 "FlashAttention does not support fp8 kv-cache on this device.")
 
+    @staticmethod
+    def _get_causal_option(attn_type: str) -> bool:
+        """
+        Determine whether the given attention type is suitable for causal 
+        attention mechanisms.
+
+        Args:
+            attn_type (AttentionType): The type of attention being evaluated
+
+        Returns:
+            bool: Returns `True` if the attention type is suitable for causal 
+            attention (i.e., not encoder, encoder-only, or encoder-decoder), 
+            otherwise returns `False`.
+        """
+        return not (attn_type == AttentionType.ENCODER
+                    or attn_type == AttentionType.ENCODER_ONLY
+                    or attn_type == AttentionType.ENCODER_DECODER)
+
     def forward(
         self,
         layer: torch.nn.Module,
@@ -428,6 +470,14 @@ def forward(
             # Profiling run.
             return output
 
+        # Validate attention metadata based on attention type
+        attn_type = self.attn_type
+        if (attn_type in (AttentionType.ENCODER, AttentionType.ENCODER_DECODER,
+                          AttentionType.ENCODER_ONLY)
+                and (not attn_metadata.is_all_encoder_attn_metadata_set)):
+            raise AttributeError("Encoder attention requires setting "
+                                 "encoder metadata attributes.")
+
         # IMPORTANT!
         # NOTE(woosuk): With piece-wise CUDA graphs, this method is executed in
         # eager-mode PyTorch. Thus, we need to be careful about any CPU overhead
@@ -438,22 +488,40 @@ def forward(
         # performance to make sure it does not introduce any overhead.
 
         num_actual_tokens = attn_metadata.num_actual_tokens
+
+        # Handle encoder attention differently - no KV cache needed
+        if attn_type == AttentionType.ENCODER:
+            # For encoder attention,
+            # we use direct Q, K, V tensors without caching
+            return self._forward_encoder_attention(query[:num_actual_tokens],
+                                                   key[:num_actual_tokens],
+                                                   value[:num_actual_tokens],
+                                                   output[:num_actual_tokens],
+                                                   attn_metadata, layer)
+
+        # For decoder and cross-attention, use KV cache as before
         key_cache, value_cache = kv_cache.unbind(0)
 
-        if self.kv_sharing_target_layer_name is None:
+        if (self.kv_sharing_target_layer_name is None and (key is not None)
+                and (value is not None)):
             # Reshape the input keys and values and store them in the cache.
             # Skip this if sharing KV cache with an earlier attention layer.
             # NOTE(woosuk): Here, key and value are padded while slot_mapping is
             # not padded. However, we don't need to do key[:num_actual_tokens]
             # and value[:num_actual_tokens] because the reshape_and_cache_flash
             # op uses the slot_mapping's shape to determine the number of
             # actual tokens.
+            if attn_type == AttentionType.ENCODER_DECODER:
+                updated_slot_mapping = attn_metadata.cross_slot_mapping
+            else:
+                updated_slot_mapping = attn_metadata.slot_mapping
+
             reshape_and_cache_flash(
                 key,
                 value,
                 key_cache,
                 value_cache,
-                attn_metadata.slot_mapping,
+                updated_slot_mapping,
                 self.kv_cache_dtype,
                 layer._k_scale,
                 layer._v_scale,
@@ -477,7 +545,7 @@ def forward(
             block_table = attn_metadata.block_table
             scheduler_metadata = attn_metadata.scheduler_metadata
 
-            descale_shape = (cu_seqlens_q.shape[0] - 1, key.shape[1])
+            descale_shape = (cu_seqlens_q.shape[0] - 1, self.num_kv_heads)
 
             flash_attn_varlen_func(
                 q=query[:num_actual_tokens],
@@ -489,7 +557,7 @@ def forward(
                 seqused_k=seqused_k,
                 max_seqlen_k=max_seqlen_k,
                 softmax_scale=self.scale,
-                causal=True,
+                causal=FlashAttentionImpl._get_causal_option(attn_type),
                 alibi_slopes=self.alibi_slopes,
                 window_size=self.sliding_window,
                 block_table=block_table,
@@ -524,12 +592,86 @@ def forward(
             fa_version=self.vllm_flash_attn_version,
             prefix_scheduler_metadata=attn_metadata.prefix_scheduler_metadata,
             suffix_scheduler_metadata=attn_metadata.scheduler_metadata,
-            q_descale=layer._q_scale,
-            k_descale=layer._k_scale,
-            v_descale=layer._v_scale,
+            q_descale=layer._q_scale.expand(descale_shape),
+            k_descale=layer._k_scale.expand(descale_shape),
+            v_descale=layer._v_scale.expand(descale_shape),
         )
         return output
 
+    def _forward_encoder_attention(
+        self,
+        query: torch.Tensor,
+        key: torch.Tensor,
+        value: torch.Tensor,
+        output: torch.Tensor,
+        attn_metadata: FlashAttentionMetadata,
+        layer: torch.nn.Module,
+    ) -> torch.Tensor:
+        """Forward pass for encoder attention without KV cache.
+
+        Args:
+            query: shape = [num_encoder_tokens, num_heads, head_size]
+            key: shape = [num_encoder_tokens, num_kv_heads, head_size]
+            value: shape = [num_encoder_tokens, num_kv_heads, head_size]
+            output: shape = [num_encoder_tokens, num_heads, head_size]
+            attn_metadata: Encoder attention metadata
+            layer: The attention layer
+        """
+        # For encoder attention, process FP8 quantization if needed
+        if self.kv_cache_dtype.startswith("fp8"):
+            num_tokens, num_heads, head_size = query.shape
+            query, _ = ops.scaled_fp8_quant(
+                query.reshape(
+                    (num_tokens, num_heads * head_size)).contiguous(),
+                layer._q_scale)
+            query = query.reshape((num_tokens, num_heads, head_size))
+
+            num_kv_tokens, num_kv_heads, head_size = key.shape
+            key, _ = ops.scaled_fp8_quant(
+                key.reshape(
+                    (num_kv_tokens, num_kv_heads * head_size)).contiguous(),
+                layer._k_scale)
+            key = key.reshape((num_kv_tokens, num_kv_heads, head_size))
+
+            value, _ = ops.scaled_fp8_quant(
+                value.reshape(
+                    (num_kv_tokens, num_kv_heads * head_size)).contiguous(),
+                layer._v_scale)
+            value = value.reshape((num_kv_tokens, num_kv_heads, head_size))
+
+        # Use encoder-specific metadata for sequence information
+        cu_seqlens_q = attn_metadata.encoder_seq_start_loc
+        cu_seqlens_k = attn_metadata.encoder_seq_start_loc
+        max_seqlen_q = attn_metadata.max_encoder_seq_len
+        max_seqlen_k = attn_metadata.max_encoder_seq_len
+
+        descale_shape = (
+            cu_seqlens_q.shape[0] - 1,  # type: ignore[union-attr]
+            self.num_kv_heads)
+
+        # Call flash attention directly on Q, K, V tensors
+        flash_attn_varlen_func(
+            q=query,
+            k=key,
+            v=value,
+            out=output,
+            cu_seqlens_q=cu_seqlens_q,
+            cu_seqlens_k=cu_seqlens_k,
+            max_seqlen_q=max_seqlen_q,
+            max_seqlen_k=max_seqlen_k,
+            softmax_scale=self.scale,
+            causal=False,  # Encoder attention is bidirectional
+            alibi_slopes=self.alibi_slopes,
+            window_size=self.sliding_window,
+            softcap=self.logits_soft_cap,
+            fa_version=self.vllm_flash_attn_version,
+            q_descale=layer._q_scale.expand(descale_shape),
+            k_descale=layer._k_scale.expand(descale_shape),
+            v_descale=layer._v_scale.expand(descale_shape),
+        )
+
+        return output
+
 
 def use_cascade_attention(
     common_prefix_len: int,

vllm/v1/attention/backends/utils.py

@@ -59,6 +59,14 @@ class CommonAttentionMetadata:
     block_table_tensor: torch.Tensor
     slot_mapping: torch.Tensor
 
+    # Encoder/cross-attention specific fields (optional)
+    encoder_seq_start_loc: Optional[torch.Tensor] = None
+    """(batch_size + 1,), cumulative encoder sequence lengths"""
+    max_encoder_seq_len: Optional[int] = None
+    """Maximum encoder sequence length in batch"""
+    cross_slot_mapping: Optional[torch.Tensor] = None
+    """Slot mapping for cross-attention KV cache"""
+
     def __post_init__(self):
         # Fill unused with -1. Needed for reshape_and_cache in full cuda graph
         # mode.