[Thisish3]

Purpose

On RDNA (gfx12), VLLM_ROCM_USE_AITER=1 always puts ROCM_AITER_UNIFIED_ATTN first, and
VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION=0 has no effect there. That leaves no way to avoid
this one kernel without turning off aiter entirely (and losing its other Triton paths such as
linear GEMM).

The original LDS-overflow crash from #56021 is fixed on the kernel side by ROCm/aiter#4868
(shipped in aiter v0.1.23), and AMD selects this backend on gfx12 by default on purpose. So
this PR is now deliberately narrow:

• Default unchanged: with the variable unset, RDNA still prefers ROCM_AITER_UNIFIED_ATTN.
• Explicit opt-out honored: VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION=0 drops only this
  backend on RDNA. This matters while the kernel does not support multimodal inputs and
  silently produces wrong attention output for them (per AMD in [Bug]: On RDNA (gfx12), VLLM_ROCM_USE_AITER=1 force-selects ROCM_AITER_UNIFIED_ATTN at priority 0, ignoring VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION (kernel exceeds RDNA's LDS limit) #56021).

is_rdna_unified_attn_enabled() uses envs.is_set() to tell "unset" from "explicitly 0".

Related: #56021

Not a duplicate

Searched open PRs for 56021 in:body, ROCM_AITER_UNIFIED_ATTN RDNA,
VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION, gfx12 unified attention and rdna4 aiter attention.
None changes how this variable is read on RDNA. #61332 and #42798 also edit
_get_backend_priorities(), but for the CDNA ordering of ROCM_AITER_FA / ROCM_ATTN, and keep
the RDNA branch as is; whichever lands second needs a trivial rebase. ROCm/aiter#4868 fixes the
kernel's LDS budget and does not touch vLLM's backend selection.

Test Plan

• ruff check and ruff format --check (ruff 0.14.0, as pinned in pre-commit): pass.
• tests/v1/attention/test_rocm_attention_backends_selection.py::test_rdna_unified_attn_backend_opt_out
  covers unset / 0 / 1 through the real env handling (refresh_env_variables() +
  _get_backend_priorities()), mocking only the RDNA/aiter detection.
• Not run locally: this file is ROCm-only (module-level skip), and I do not have a ROCm
  device. I'd appreciate an /amd-ci run.

No model output changes when the variable is unset (default path unchanged), so no eval was run.

AI assistance

This change was written with AI assistance (Claude Code). I reviewed every changed line and
can explain and defend the change.

[claude]

Claude Code Review

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[amd-xavierwang]

Hello. I have issued a fix to ROCm/aiter#4868 and we are working on optimizing this backend for gfx1201 in the future.

[Thisish3]

@amd-xavierwang thanks for ROCm/aiter#4868, that fixes the crash from #56021 at the source.

I've narrowed this PR accordingly: the RDNA default (prefer ROCM_AITER_UNIFIED_ATTN when aiter
is on) is left as is, and the only change is that an explicit
VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION=0 now opts out of this one backend. Given your note that
the kernel doesn't support multimodal inputs yet and returns wrong output for them, that seems
like a useful escape hatch that keeps aiter's other Triton paths on.

The test is ROCm-only so I couldn't run it locally. Could someone with access trigger
/amd-ci run?

[Thisish3]

Hi @amd-xavierwang, could someone from the ROCm side take a look at this when you have time? After ROCm/aiter#4868 fixed the crash at the source, I narrowed the PR to one change: an explicit VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION=0 is now honored on RDNA. The RDNA default is unchanged. Thanks!

[amd-xavierwang]

Thanks for the contribution. I took a look at the change; it makes sense to honour this additional flag to disable AITER attention. However, there is an alternative to use --attention-backend TRITON_ATTN for example to ultimately overwrite attention backends, and I believe this mechanism should be encouraged to make dispatch consistent and avoid any confusions.

As a result, just a little bit concern for this PR adding additional complexity to backend selection. The complexity is already very horrible with cls._TRITON_UNIFIED_ATTN_ENABLED, VLLM_USE_AITER, rocm_aiter_ops.is_rdna_aiter_enabled, ... How about we deprecate the flag and always use --attention-backend to overwrite? what do you think?

[Thisish3]

Thanks for taking a look, that makes sense. I checked get_attn_backend_cls: a backend chosen with --attention-backend is validated and used before the priority list, so --attention-backend TRITON_ATTN already avoids the kernel from #56021 on RDNA, and ROCm/aiter#4868 fixed the crash itself. Adding another branch for the env var isn't worth the extra complexity, so I'll close this PR. If you decide to deprecate VLLM_ROCM_USE_AITER_UNIFIED_ATTENTION in favor of --attention-backend, I'm happy to help with that change. Thanks again!