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[Bugfix] Clarify FLASHINFER limitation for per-attention-head KV quantization - #40448

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MerIinnn wants to merge 2 commits into
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MerIinnn:bugfix/flashinfer-attn-head-kv-quant
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MerIinnn wants to merge 2 commits into
vllm-project:mainfrom
MerIinnn:bugfix/flashinfer-attn-head-kv-quant

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Fixes #40444

Summary

  • raise a clear validation error when per-attention-head KV-cache quantization is requested with the FLASHINFER backend
  • add regression coverage for the FLASHINFER failure path while keeping the existing FLASH_ATTN success path
  • document that per-attention-head KV-cache quantization currently requires FLASH_ATTN

Why this is not duplicate work

  • I checked issue/PR state for #40444 and did not find an open PR that already addresses this fix
  • this PR does not add FlashInfer kernel support; it makes the current backend limitation explicit and fail-fast

Test plan

  • uv run --no-project python -m py_compile vllm/v1/attention/backend.py tests/test_attention_backend_registry.py tests/quantization/test_compressed_tensors.py
  • ./.venv/bin/python -m pytest tests/test_attention_backend_registry.py -k per_head_quant_scale_support -v
  • ./.venv/bin/python -m pytest tests/quantization/test_compressed_tensors.py -k per_attn_head -v

AI assistance

  • This PR was prepared with AI assistance, and the final changes were reviewed by a human submitter.

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Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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@mergify

mergify Bot commented Apr 21, 2026

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Documentation preview: https://vllm--40448.org.readthedocs.build/en/40448/

@mergify mergify Bot added documentation Improvements or additions to documentation v1 bug Something isn't working labels Apr 21, 2026

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Code Review

This pull request updates the documentation and validation logic to clarify that per-attention-head KV-cache quantization is currently only supported with the FLASH_ATTN backend. It also adds tests to verify that using FLASHINFER with this feature correctly raises a ValueError. Feedback was provided regarding the test implementation, specifically that calling get_class() during test parameterization could cause import errors on non-CUDA platforms. Additionally, it was noted that calling the abstract method get_name() in the base class's validation logic could lead to a NotImplementedError and violates modularity by hardcoding backend names.

Comment thread tests/test_attention_backend_registry.py
Comment thread vllm/v1/attention/backend.py Outdated
@MerIinnn
MerIinnn requested a review from vadiklyutiy as a code owner April 21, 2026 09:40
@mergify mergify Bot added the nvidia label Apr 21, 2026
@MerIinnn
MerIinnn force-pushed the bugfix/flashinfer-attn-head-kv-quant branch from 11408c7 to f487271 Compare April 21, 2026 10:48
merlin added 2 commits April 21, 2026 18:50
…tization

Raise a clear validation error when per-attention-head KV-cache quantization is
requested with the FLASHINFER backend, add regression coverage for the
FLASHINFER failure path, and document that this mode currently requires
FLASH_ATTN.

Signed-off-by: merlin <mer1inn@163.com>
Resolve FLASHINFER lazily in the backend registry test to avoid import-time
failures during collection, and move the FLASHINFER-specific per-head KV
quantization validation message out of the attention backend base class.

Signed-off-by: merlin <mer1inn@163.com>
@MerIinnn
MerIinnn force-pushed the bugfix/flashinfer-attn-head-kv-quant branch from f487271 to e061d74 Compare April 21, 2026 10:50
@MerIinnn

MerIinnn commented Apr 21, 2026

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@zhuohan123 @youkaichao Hi, I'm a new contributor. Could you please add the ready label to trigger CI? Thank you!

@mergify

mergify Bot commented Jun 23, 2026

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This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @MerIinnn.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

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