[Bugfix][DCP] Fix NVIDIA DeepSeek-V3.2 / GLM-5.2 fused attention

[foraxe]

[Bugfix][DCP] Fix NVIDIA DeepSeek-V3.2 / GLM-5.2 fused attention

Fixes #50095.

Purpose

Fix two DCP correctness defects in the fused DeepSeek-V3.2 / GLM-5.2
attention path:

• The fused norm/RoPE kernel skipped query RMSNorm when a DCP rank had no
  owner-local KV-cache slot. KV ownership does not remove that rank's query
  contribution.
• Pure DCP sent only rank-local query heads into attention over owner-local KV
  and did not combine the resulting partial output/LSE across DCP ranks.

The sparse-indexer DCP rank, world-size, and interleave plumbing described in
the original version of this PR has since landed independently on main; this
PR no longer changes that part.

Implementation

• Run query RMSNorm before the owner-local slot check. Negative slots still
  suppress MLA and indexer KV-cache writes.
• For pure DCP, gather query heads through MLADCPManager before sparse MLA
  attention. The manager keeps the direct-workspace fast path when supported
  and falls back safely for oversized mixed batches.
• Combine partial output/LSE through the existing MLADCPManager after
  attention. For this NVIDIA pure-DCP mixed-batch path, pass no PCP-only decode
  metadata; its sparse backend supplies neutral contributions for empty local
  shards.
• Extend the existing fused norm/RoPE test with an all-negative-slot regression
  that checks valid query output, unchanged caches, and Top-K clearing.

Non-duplication

I re-audited related open work on August 23, 2026. Searching for issue #50095
finds only this PR.

• [Attention][MLA] Add GLM-5.2 TurboQuant sparse backend with DCP/MTP #52472 adds a separate TurboQuant sparse MLA backend and does not modify this
  fused DeepSeek-V3.2 attention or norm/RoPE path.
• [Bugfix][DCP] Handle sparse MLA metadata after DCP Manager refactor #52377 has landed with generic sparse-MLA forced-MQA metadata and
  direct-workspace fallback handling after the DCP-manager refactor; this PR
  now consumes that manager path but still supplies the NVIDIA fused override
  and negative-slot query-normalization fixes.
• [Bugfix][SM120][MLA] Enable DCP for FlashInfer sparse MLA decode #47779 enables DCP in the SM120 FlashInfer sparse MLA wrapper.
• [DCP][Performance] Add Shared-DCP peer-addressable decode data paths #50009 is independent Shared-DCP performance work.

None fixes both live defects in
vllm/models/deepseek_v32/{attention.py,common/kernels.py}.

Validation

Current upstream base inspected: b2dd9ce73d
Current PR head: 22e9a19f8d

The branch was forward-ported across the August DeepSeek-V3.2 file moves by
@drakosha in foraxe#1. Against current main, the PR contains only the
three intended files. git diff --check passes.

On one GB200 GPU at the exact PR head:

CUDA_VISIBLE_DEVICES=0 PYTHONPATH=$PWD \
  /workspace/vllm_cp/vllm/.venv/bin/python -m pytest \
  tests/kernels/test_fused_deepseek_v32_norm_rope.py -q
# 67 passed

All applicable changed-file pre-commit hooks pass, including Ruff, formatting,
mypy, SPDX, forbidden-import, and accelerator-API checks.

Independent forward-port validation by @drakosha used 4xH200 with
GLM-5.2-NVFP4, TP4/DCP4, fp8_ds_mla, and greedy decoding. On the tested
prompt, patched DCP4 output was byte-identical to DCP1; unpatched main
produced repeated-token output. The focused kernel suite passed 67 tests with
the fix, while the new negative-slot regression was the only failure without
it.

AI assistance and human review

OpenAI Codex assisted with investigation, implementation, validation
orchestration, forward-port review, and drafting. @drakosha reports AI
assistance for the forward port and H200 validation.

• The submitter has reviewed every changed line, verified the reported
  local evidence, and can explain and defend the query-gather and output/LSE
  merge protocol.

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[foraxe]

Tracking issue: #50095.

#50005 is the proposed fix for the NVIDIA DeepSeek-V3.2 / GLM-5.2 fused path bypassing DCP query normalization, sparse-indexer geometry, query-head gathering, and partial output/LSE merging.

This correctness work is independent of the experimental Shared-DCP performance feature.

[claude]

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.

[mergify]

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

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

[drakosha]

@foraxe this fix is still needed. Both defects are live on main as of da329cc30: the early return on slot_mapping < 0 in common/kernels.py still skips the pid == 2 query RMSNorm, and the head gather plus cross-rank LSE merge in attention.py are still behind use_pcp. GLM-5.2 with plain DCP=4 produces repeated-token garbage on current main because of it.

The branch conflicts because the DSA path moved in August (deepseek_v32/nvidia/{attention,kernels}.py became deepseek_v32/attention.py and common/kernels.py). I brought your branch up to current main and opened foraxe#1 against it. It is a merge commit, so it applies without conflicts; merging it there updates this PR and drops needs-rebase. The fix is unchanged, your regression test came with it, and the commit keeps you as author.

What we can add is validation on hardware, which this PR did not have: on 4xH200 with GLM-5.2-NVFP4 (TP4 + DCP4, fp8_ds_mla) the patched build gives DCP=4 output byte-identical to DCP=1 under greedy decoding. The kernel suite passes 67 tests with the fix; without it your new case is the only failure.

If you would rather hand this off, say so and I will open a superseding PR with you as author. Otherwise it is yours, and I will leave it a week before doing anything else with it.

AI assistance was used for the rebase and the runs.

[foraxe]

The forward-port from @drakosha has been reviewed and merged. Current head ffeea6f3d5 is mergeable against current main and contains only the three intended files. On GB200, the focused fused norm/RoPE suite passes all 67 tests, and all applicable changed-file pre-commit hooks pass. I refreshed this PR and #50095 to the current two-defect scope; indexer DCP geometry has already landed independently. The visible pre-run-check failure is only the CI authorization gate because this PR does not yet have a maintainer-applied ready or verified label. @GirasoleY @WoosukKwon, since you recently worked on this refactored path, a review would be appreciated.

[cjackal]

It seems the changes in vllm/models/deepseek_v32/attention.py is a dsv32-localized version of #52377 (As commented in this PR, sparse MLA metadata has no .decode attribute, and sparse MLA mixed batches use dcp_manager.query_gather with long prefills, both of this and mine fixes by fallback to allgather).

I don't know model archs using sparse MLA other than DSV32/DSV4, so either change makes sense I guess, just want to mention it FYI.

[drakosha]

@cjackal we can settle the "either change" question with a run. On 4xH200 with GLM-5.2-NVFP4 (TP4 + DCP4, fp8_ds_mla): nightly ba07e4a48 + #52377 alone still emits repeated-token garbage at DCP=4; nightly + this PR gives output byte-identical to DCP=1 under greedy.

They are not interchangeable because of routing: since #52861 those two archs go through vllm/models/deepseek_v32/ on NVIDIA, so the call sites #52377 fixes are no longer executed there. Your PR still covers the generic path (ROCm keeps the compiled route), so both are needed, one per path.

Your dcp_manager.query_gather point matches what we hit independently: its workspace is decode-sized while a sparse mixed batch carries prefill rows, so the allgather fallback is right.

AI assistance was used for the runs.

[cjackal]

They are not interchangeable because of routing: since #52861 those two archs go through vllm/models/deepseek_v32/ on NVIDIA, so the call sites #52377 fixes are no longer executed there. Your PR still covers the generic path (ROCm keeps the compiled route), so both are needed, one per path.

Thank you for the pointer! I missed the recent MRV2 forward override indeed, my .decode patch no longer works unless users explicitly set VLLM_USE_MODEL_RUNNER_V2=0.

[LucasWilkinson]

Thanks for the contribution! left a couple nits hopefully you can address before landing