[ROCM][Bugfix] Deferring to TRITON_ATTN backend for Prefix lm models instead of ROCM_ATTN and fixed minor bug in Gemma3 Multimodal code by GodRishUniverse · Pull Request #46754 · vllm-project/vllm

GodRishUniverse · 2026-06-25T18:16:03Z

Purpose

Fixes incorrect attention behavior for prefix-LM multimodal models (e.g., Gemma3) on ROCm. This is a correctness fix.

Root cause: Both RocmAttentionBackend and RocmAiterUnifiedAttentionBackend overrode
supports_mm_prefix() to return True, but neither backend actually implements the
mm_prefix_range_tensor passing required for bidirectional image-token attention during prefill. As a result,
prefix-LM multimodal models on ROCm were silently falling through with wrong attention masks and
image tokens were not attending to each other bidirectionally as required by models like Gemma3.

Changes:

rocm_attn.py / rocm_aiter_unified_attn.py: Removed the incorrect
supports_mm_prefix() -> True overrides. Both backends now inherit the base-class default
(False), causing the attention backend selector to correctly defer to Triton unified
attention for prefix-LM models on ROCm.
gemma3_mm.py: Explicitly sets config.text_config.is_mm_prefix_lm = True in the
multimodal constructor, since the text-only Gemma3 path does not set this flag but the
multimodal path requires it for correct image-token attention.
rocm.py: Ports the disable_chunked_mm_input guard from the CUDA platform path to
ROCm. For prefix-LM multimodal models, chunked prefill can split a multimodal item and
break the intended bidirectional attention pattern; this ensures ROCm disables it
automatically, matching CUDA behavior.
tests/kernels/attention/test_rocm_attention_selector.py: Adds a regression test
asserting that both ROCm backends do not claim mm_prefix support, guarding against this
being re-introduced silently.

Test Plan

Unit test (ROCm):

.venv/bin/python -m pytest tests/kernels/attention/test_rocm_attention_selector.py \
  -v -k test_rocm_backends_do_not_support_mm_prefix

lm-eval benchmarks ran are as follows:

Text-only tasks (lmeval) - HellaSwag, ARC-Easy, ARC-Challenge, PIQA, BoolQ, WinoGrande (Gemma3 4B and 12B):

# 4B
python -m lm_eval \
    --model vllm \
    --model_args "pretrained=google/gemma-3-4b-it,dtype=bfloat16,max_model_len=2048,tensor_parallel_size=1,gpu_memory_utilization=0.85" \
    --tasks hellaswag,arc_easy,arc_challenge,piqa,boolq,winogrande \
    --batch_size 16

# 12B
python -m lm_eval \
    --model vllm \
    --model_args "pretrained=google/gemma-3-12b-it,dtype=bfloat16,max_model_len=2048,tensor_parallel_size=1,gpu_memory_utilization=0.95" \
    --tasks hellaswag,arc_easy,arc_challenge,piqa,boolq,winogrande \
    --batch_size 16

Multimodal task - MMMU-val:

# 4B
python -m lm_eval \
    --model vllm-vlm \
    --model_args "pretrained=google/gemma-3-4b-it,dtype=bfloat16,max_model_len=4096,max_images=1,tensor_parallel_size=1,gpu_memory_utilization=0.85" \
    --tasks mmmu_val \
    --batch_size 4 \
    --apply_chat_template

# 12B
python -m lm_eval \
    --model vllm-vlm \
    --model_args "pretrained=google/gemma-3-12b-it,dtype=bfloat16,max_model_len=4096,max_images=1,tensor_parallel_size=1,gpu_memory_utilization=0.95" \
    --tasks mmmu_val \
    --batch_size 4 \
    --apply_chat_template

Test Result

No regression observed here. Hardware used was AMD Radeon AI PRO R9700

Text benchmarks

Gemma3-4B:

Text benchmarks

Gemma3-4B:

Task	Baseline (before fix)	With fix	Delta
arc_challenge	0.5341 ± 0.0146	0.5324 ± 0.0146	-0.0017
arc_easy	0.8249 ± 0.0078	0.8237 ± 0.0078	-0.0012
boolq	0.8410 ± 0.0064	0.8407 ± 0.0064	-0.0003
hellaswag	0.5583 ± 0.0050	0.5591 ± 0.0050	+0.0008
piqa	0.7644 ± 0.0099	0.7639 ± 0.0099	-0.0005
winogrande	0.6969 ± 0.0129	0.6938 ± 0.0130	-0.0031

Gemma3-12B:

Task	Baseline (before fix)	With fix	Delta
arc_challenge	0.6092 ± 0.0143	0.6118 ± 0.0142	+0.0026
arc_easy	0.8342 ± 0.0076	0.8350 ± 0.0076	+0.0008
boolq	0.8746 ± 0.0058	0.8743 ± 0.0058	-0.0003
hellaswag	0.6270 ± 0.0048	0.6271 ± 0.0048	+0.0001
piqa	0.8036 ± 0.0093	0.8003 ± 0.0093	-0.0033
winogrande	0.7451 ± 0.0122	0.7482 ± 0.0122	+0.0031

Multimodal benchmark (MMMU-val)

Model	Baseline (before fix)	With fix	Delta
Gemma3-4B-IT	0.4067 ± 0.0157	0.4133 ± 0.0158	+0.0066
Gemma3-12B-IT	0.4856 ± 0.0159	0.4922 ± 0.0158	+0.0066

Edit: 24 July 2026

Reproducing the benchmarks

Additional multimodal tasks (lmms-eval) - DocVQA, InfoVQA, AI2D, ChartQA, VQAv2 (Gemma3 4B and 12B):

# 4B
python -m lmms_eval eval \
    --model vllm \
    --model_args "model=google/gemma-3-4b-it,dtype=bfloat16,max_model_len=4096,tensor_parallel_size=1,gpu_memory_utilization=0.85" \
    --tasks ai2d,chartqa,docvqa_val,infovqa_val,vqav2_val \
    --batch_size 8 \
    --limit 1000 \
    --seed 0

# 12B
python -m lmms_eval eval \
    --model vllm \
    --model_args "model=google/gemma-3-12b-it,dtype=bfloat16,max_model_len=4096,tensor_parallel_size=1,gpu_memory_utilization=0.95" \
    --tasks ai2d,chartqa,docvqa_val,infovqa_val,vqav2_val \
    --batch_size 8 \
    --limit 1000 \
    --seed 0

Note: the lmms-eval==0.7.2 wheel ships without the _default_template_*_yaml task templates, so install it editable from the tag:
git clone --branch v0.7.2 https://github.com/EvolvingLMMs-Lab/lmms-eval && pip install -e ./lmms-eval --no-deps && pip install decord Levenshtein

Multimodal benchmarks (lmms-eval)

Multimodal accuracy improves on every text-in-image task.

Gemma3-4B:

Task	Baseline (before fix)	With fix	Delta
ai2d (exact_match)	0.6750 ± 0.0148	0.7280 ± 0.0141	+0.0530
chartqa (relaxed_overall)	0.2850 ± 0.0143	0.3480 ± 0.0151	+0.0630
docvqa_val (anls)	0.5938 ± 0.0143	0.7124 ± 0.0131	+0.1186
infovqa_val (anls)	0.3462 ± 0.0139	0.4046 ± 0.0144	+0.0584
vqav2_val (exact_match)	0.5720 ± 0.0148	0.5799 ± 0.0147	+0.0079

Gemma3-12B:

Task	Baseline (before fix)	With fix	Delta
ai2d (exact_match)	0.7810 ± 0.0131	0.8030 ± 0.0126	+0.0220
chartqa (relaxed_overall)	0.4040 ± 0.0155	0.4470 ± 0.0157	+0.0430
docvqa_val (anls)	0.6995 ± 0.0134	0.7857 ± 0.0120	+0.0862
infovqa_val (anls)	0.4456 ± 0.0145	0.5123 ± 0.0148	+0.0667
vqav2_val (exact_match)	0.6397 ± 0.0143	0.6270 ± 0.0145	-0.0127

…m_aiter_unified_attn.py as rocm attn backend does not support it and even though mm_prefix_range_tensor is built - it is not passed down so we need to defer to triton attn backend to be able to serve prefix lm models properly Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com>

Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com>

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github-actions · 2026-06-25T18:16:13Z

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GodRishUniverse · 2026-06-25T18:23:35Z

Hi, could a maintainer please add the ready label to trigger CI? I am a new contributor here. Thank you.

GodRishUniverse · 2026-07-03T13:44:29Z

Hey @tjtanaa @AndreasKaratzas @dllehr-amd ! Wanted to check in if you guys were able to go over the PR. Thanks!

Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com>

GodRishUniverse · 2026-07-09T14:10:46Z

Hey @tjtanaa @dllehr-amd ! Could I please get a review for this PR? I would greatly appreciate it.

Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com>

GodRishUniverse · 2026-07-20T14:10:51Z

Follow up: @tjtanaa @dllehr-amd ! Any updates? Thanks!

mergify · 2026-10-06T14:59:26Z

Hi @GodRishUniverse, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

mergify · 2026-10-06T15:08:47Z

Hi @GodRishUniverse, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
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For future commits, pre-commit will run automatically on changed files before each commit.

Signed-off-by: GodRishUniverse <risagarw@amd.com>

amd-xavierwang · 2026-10-06T17:18:51Z

/ci run

github-actions · 2026-10-06T17:19:10Z

❌ @amd-xavierwang, Only reviewers with write access can use CI commands before CI is delegated to the PR author.

GodRishUniverse · 2026-10-06T17:48:45Z

/ci run

github-actions · 2026-10-06T17:49:06Z

❌ This PR is 2 commits behind upstream main. Your branch must contain every commit currently on upstream main. No new CI build was started. Merge or rebase onto the latest main, then rerun /ci run. To test this branch at your own risk, use /ci run --allow-stale.

GodRishUniverse · 2026-10-06T17:49:23Z

/ci run --allow-stale

github-actions · 2026-10-06T17:49:44Z

✅ Triggered Buildkite CI #93145 for commit f0bcb79cdfd5.

⚠️ This PR is 2 commits behind upstream main. Running CI at your own risk because --allow-stale was requested; outdated CI configuration may cause failures. Before merging, merge or rebase onto the latest main, then rerun /ci run on the latest PR commit.

vllm-agent · 2026-10-06T17:55:06Z

CI selector (shadow): 8 test steps (30 jobs) instead of 63 (97 jobs)

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_{2 changed files · base b9767299db · head 60aae201b2 · Python record: build 93039 at 1e5d0ea888 · kernel record: table 43b4aae (build 93244), map 43b4aae · not counted: 10 build steps, 5 A100 steps the generator no longer emits, 13 optional steps the selector would also run}

GodRishUniverse · 2026-10-06T17:57:01Z

/ci run

github-actions · 2026-10-06T17:57:23Z

✅ Triggered Buildkite CI #93147 for commit 80b483d99708.

mergify · 2026-10-06T18:04:50Z

Hi @GodRishUniverse, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
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Then, commit the changes and push to your branch.

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GodRishUniverse · 2026-10-07T13:56:16Z

/ci run

github-actions · 2026-10-07T13:56:37Z

✅ Triggered Buildkite CI #93321 for commit 90eac93e95a2.

GodRishUniverse · 2026-10-07T19:08:28Z

/ci run

github-actions · 2026-10-07T19:09:51Z

✅ Triggered Buildkite CI #93389 for commit 60aae201b21d.

GodRishUniverse · 2026-10-08T13:32:01Z

/ci run

github-actions · 2026-10-08T13:32:25Z

✅ Triggered Buildkite CI #93599 for commit 2db54d49f296.

…instead of ROCM_ATTN and fixed minor bug in Gemma3 Multimodal code (vllm-project#46754) Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com> Signed-off-by: Rishabh Agarwal <risagarw@amd.com> Signed-off-by: GodRishUniverse <risagarw@amd.com> Signed-off-by: Andreas Karatzas <AndreasKaratzas@users.noreply.github.com>

…instead of ROCM_ATTN and fixed minor bug in Gemma3 Multimodal code (vllm-project#46754) Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com> Signed-off-by: Rishabh Agarwal <risagarw@amd.com> Signed-off-by: GodRishUniverse <risagarw@amd.com>

GodRishUniverse added 3 commits June 25, 2026 15:49

[ROCm] Fix prefix-LM attention backend selection and add unit tests

5a6890c

Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com>

removed a note comment added

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Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com>

GodRishUniverse requested review from AndreasKaratzas, dllehr-amd and tjtanaa as code owners June 25, 2026 18:16

claude Bot reviewed Jun 25, 2026

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mergify Bot added rocm Related to AMD ROCm v1 bug Something isn't working labels Jun 25, 2026

github-project-automation Bot added this to AMD Jun 25, 2026

github-project-automation Bot moved this to Todo in AMD Jun 25, 2026

Merge branch 'main' into rocm_backend_prefix_lm_fix

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GodRishUniverse added 3 commits June 25, 2026 19:38

Merge branch 'main' into rocm_backend_prefix_lm_fix

5fdc48e

Merge branch 'main' into rocm_backend_prefix_lm_fix

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Merge branch 'main' into rocm_backend_prefix_lm_fix

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AndreasKaratzas reviewed Jul 3, 2026

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Comment thread tests/kernels/attention/test_rocm_attention_selector.py Resolved

Comment thread tests/kernels/attention/test_rocm_attention_selector.py Resolved

Comment thread vllm/v1/attention/backends/rocm_aiter_unified_attn.py Resolved

GodRishUniverse and others added 2 commits July 6, 2026 13:53

removed strong 'must' and also added file wide skip for

e3b4101

Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com>

Merge branch 'main' into rocm_backend_prefix_lm_fix

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Merge branch 'main' into rocm_backend_prefix_lm_fix

4c5078a

This was referenced Jul 9, 2026

[Feature]: Missing Prefix LM kernel for Multimodal Range Tensors ROCm/aiter#4168

Open

[Feature]: Prefix LM kernel on ROCm_ATTN #48172

Open

GodRishUniverse and others added 3 commits July 10, 2026 14:31

refactored assertion messaged to include the issues created

937b726

Signed-off-by: GodRishUniverse <rishabh.agarwal@amd.com>

Merge branch 'main' into rocm_backend_prefix_lm_fix

2f22a0c

Merge branch 'main' into rocm_backend_prefix_lm_fix

3fb2704

Merge branch 'main' into rocm_backend_prefix_lm_fix

8dabe4a

Fix ruff format

d53b2e6

Signed-off-by: GodRishUniverse <risagarw@amd.com>

Merge branch 'main' into rocm_backend_prefix_lm_fix

f0bcb79

Merge branch 'main' into rocm_backend_prefix_lm_fix

80b483d

Merge branch 'main' into rocm_backend_prefix_lm_fix

90eac93

nemanjaudovic mentioned this pull request Oct 7, 2026

[ROCm] [RDNA] [Perf] Add tuned configs for TRITON_ATTN kernel and swap places of TRITON_ATTN and ROCM_ATTN in priority list #60418

Merged

Merge branch 'main' into rocm_backend_prefix_lm_fix

60aae20

Merge branch 'main' into rocm_backend_prefix_lm_fix

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dllehr-amd merged commit f395a6b into vllm-project:main Oct 13, 2026
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github-project-automation Bot moved this from Todo to Done in AMD Oct 13, 2026

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Purpose

Test Plan

Test Result

Text benchmarks

Text benchmarks

Multimodal benchmark (MMMU-val)

Edit: 24 July 2026

Reproducing the benchmarks

Multimodal benchmarks (lmms-eval)

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