[ROCm] Make Whisper causal attention backend-agnostic

[laudney]

Summary

• Remove hardcoded backend allowlist (FlashAttentionBackend, AiterFlashAttentionBackend, RocmAttentionBackend, TritonAttentionBackend) from whisper_causal.py
• Remove the corresponding explicit imports and _SUPPORTED_BACKENDS check
• The model already uses get_attn_backend from the attention selector and subclass_attention_backend_with_overrides to wrap the selected backend — the allowlist was redundant and blocked backends that work fine (e.g. on RDNA4/gfx12)

This is a pure deletion (~39 lines removed, 0 added). The subclass_attention_backend_with_overrides mechanism already validates backend compatibility at a lower level.

Test plan

• Whisper causal inference on FlashAttention (CUDA) — no behavior change
• Whisper causal inference on ROCm with non-Flash backends
• Existing CI should pass (no new code paths)

[gemini-code-assist]

Code Review

This pull request makes the Whisper causal attention backend-agnostic by removing a hardcoded allowlist of backends. This change correctly identifies that the explicit list was redundant, as backend validation is already handled by get_attn_backend. By deleting the allowlist and associated imports, the code is simplified and more maintainable, and it enables support for newer backends on platforms like ROCm without requiring modifications to this file. The changes are sound and represent a good improvement.

[DarkLight1337]

cc @tjtanaa @AndreasKaratzas can you verify this model/attention backend combination?

[laudney]

Related PRs (RDNA4/gfx12 series)

This PR is part of a series enabling RDNA4 (gfx12) support in vLLM:

• [ROCm] Enable wvSplitK skinny GEMM kernel for RDNA4/gfx1x decode #34709 — [ROCm] Enable wvSplitK/wvSplitKQ skinny GEMM kernels for RDNA4
• [ROCm] Use supports_fp8() for FP8 feature gates instead of arch checks #34740 — [ROCm] Use supports_fp8() for FP8 feature gates instead of arch checks
• [ROCm] Enable FP8 KV-cache and relax constraints for RDNA4 custom paged attention #34741 — [ROCm] Enable FP8 KV-cache for RDNA4 custom paged attention
• [ROCm] Add MXFP4 inline dequant Triton kernel for RDNA4/gfx12 #34632 — [ROCm] Add MXFP4 inline dequant Triton kernel for RDNA4/gfx12

Each PR is independent and can be reviewed/merged separately.

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[AndreasKaratzas]

cc @tjtanaa @AndreasKaratzas can you verify this model/attention backend combination?

Definitely. @laudney can you please help me save some time? Would it be easy to have all of those PRs into one branch and give me the fork and commit hash of that branch? If you do that I will certainly be able to evaluate your changes and share the results here :)

[laudney]

@AndreasKaratzas Sure! Here you go:

Fork: mmonad/vllm
Branch: feat/rocm-rdna4-combined
Commit: 0a3d6653a0d414a047904f301f9190305250c672

This branch contains all 8 commits from the 5 PRs on top of upstream/main:

1. 21c8125 — [ROCm] Make Whisper causal attention backend-agnostic ([ROCm] Make Whisper causal attention backend-agnostic #34631)
2. 9ae352a — [ROCm] Use supports_fp8() for FP8 feature gates instead of arch checks ([ROCm] Use supports_fp8() for FP8 feature gates instead of arch checks #34740)
3. 0fbf2a8 — [ROCm] Enable FP8 KV-cache and relax constraints for RDNA4 custom paged attention ([ROCm] Enable FP8 KV-cache and relax constraints for RDNA4 custom paged attention #34741)
4. 79b1817 — [ROCm] Enable LLMM1 skinny GEMM kernel for RDNA4/gfx1x decode ([ROCm] Enable wvSplitK skinny GEMM kernel for RDNA4/gfx1x decode #34709)
5. f838bfd — [ROCm] Enable wvSplitK skinny GEMM kernel for RDNA4/gfx1x decode ([ROCm] Enable wvSplitK skinny GEMM kernel for RDNA4/gfx1x decode #34709)
6. dd3c6f9 — [ROCm] Enable wvSplitKQ FP8 skinny GEMM kernel for RDNA4/gfx12 decode ([ROCm] Enable wvSplitK skinny GEMM kernel for RDNA4/gfx1x decode #34709)
7. dd153f6 — [ROCm] Add MXFP4 inline dequant Triton kernel for RDNA4/gfx12 ([ROCm] Add MXFP4 inline dequant Triton kernel for RDNA4/gfx12 #34632)
8. 0a3d665 — Fix MXFP4 dequant kernel: use MoEActivation enum instead of strings ([ROCm] Add MXFP4 inline dequant Triton kernel for RDNA4/gfx12 #34632)

Thank you for taking the time to evaluate!

[AndreasKaratzas]

Will let you know as soon as I can the results :) We are trying to address some high priority tasks first for CI to report accurately results, and I'm going to get back to you probably by tomorrow this time. Sry for delays

[laudney]

@AndreasKaratzas No worries at all, take your time! Really appreciate you looking into this. We all want better AMD support in this amazing project, so happy to help if any questions come up during testing.

[AndreasKaratzas]

@AndreasKaratzas No worries at all, take your time! Really appreciate you looking into this. We all want better AMD support in this amazing project, so happy to help if any questions come up during testing.

Full CI run is up: https://buildkite.com/vllm/amd-ci/builds/4991/steps/canvas

[AndreasKaratzas]

@laudney I am going to take a look at this again tomorrow :) Sry for the delay, have been working on some critical CI-related tasks.

[AndreasKaratzas]

Full CI build as of yesterday: https://buildkite.com/vllm/amd-ci/builds/4991/steps/canvas

There were no new regressions observed. At the same time I realize that these changes mostly affect other architectures, not gfx9. Therefore, I should ask testing on gfx11/12 as well for those. Also there are changes inside the attention.cu file which means that we should do a perf analysis of that.

cc @tjtanaa @gshtras

[laudney]

Hey, this is approved and rebased on latest main. What else do I need to do to get it merged?

[DarkLight1337]

From @AndreasKaratzas

Therefore, I should ask testing on gfx11/12 as well for those. Also there are changes inside the attention.cu file which means that we should do a perf analysis of that.

[AndreasKaratzas]

From @AndreasKaratzas

Therefore, I should ask testing on gfx11/12 as well for those. Also there are changes inside the attention.cu file which means that we should do a perf analysis of that.

I see that this PR has probably been heavily refactored. Specifically, the file that I note in my first comment (attention.cu) is not in the diff. So I think I am good with this PR as long as it is passing the tests. @laudney please rebase with latest main so that we do a comparison of failures.

[laudney]

@AndreasKaratzas Rebased on latest main. Ready for CI.