[Bugfix][SM70] Stop grouped-page4 null-block padding from leaking E4M3 NaN

[areslp]

The SM70 grouped-page4 QSA prefill route can emit NaN for every query row and head on a fixed set of head dims, but only when the KV cache is E4M3 and a hybrid (GDN/Mamba) layout co-locates non-attention state in the paged KV pool. The same batch is correct through the XQA route, so the failure looks route-specific rather than data-specific.

Root cause

The grouped planner pads each category up to a multiple of eight with the null block — (physical microblock 0, mask 0) — and counts that padding in seq_len. The forward then loads page 0's K/V for those padded rows and masks them multiplicatively: the score is -inf, so the softmax probability is P = 0.

But 0 * NaN = NaN survives the P@V tensor-core reduction. Under E4M3 the null block's bytes decode to NaN (0x7F/0xFF → fp16 0x7e00, and any fp16 state a hybrid layout happens to place at page 0 has ~0.4% of its bytes land on those encodings). Every group's every tile reads the same page-0 microblock, so the whole tile's rows and heads go NaN on identical dims.

XQA encodes unused slots as INT64_MAX, sorts them last, and only counts real tokens in its sequence length — it never reads the null block. That is why XQA (and Triton) stay clean on the same inputs, and why the bug is invisible with an all-zero page 0 (0 * 0 = 0) or with fp16 KV (the null block reads back as finite small values, 0 * finite = 0).

Fingerprint

The padded microblock is page 0's tokens 0–3 × 256 dims = 1024 value bytes. A poisoned/garbage null block turns ~0.3–0.5% of those into E4M3 NaN, and the observed NaN dims are bit-identical across all 16 rows and 6 heads of a tile — matching a single shared source region rather than per-row corruption.

Fix (defense in depth, both default-on)

Kernel — in flash_attention_grouped_verify_e5m2_partial_kernel under SPARSE_PAGE4, after each K/V panel load, zero every row that no query attends. The mask bit layout is query*4 + token_in_microblock, so a row is unattended iff (mask & (0x11111111u << (row & 3))) == 0; zeroing those rows makes the reduction compute 0 * 0 = 0. A leading __syncthreads() before the V-panel zeroing keeps the panel load from racing past the zero and resurrecting the NaN; the trailing one publishes the zeros before P@V.

Python (VLLM_SM70_QSA_GROUPED_PAD_FIX, default 1) — after the planner, repoint every mask == 0 padding entry at the group's first real microblock (column 0; real entries always carry a nonzero mask). torch.where + copy_ has no host sync, so it stays CUDA-graph capturable. This backstops any binary built before the kernel fix.

The grouped route itself stays default-on (VLLM_SM70_QSA_GROUPED_PAGE4=1); this PR only changes what the padded rows contribute.

Verification

Differential harness on V100 (SM70), 32-token physical pages, page 0 as the null block, poisoned with E4M3 NaN bytes on the value plane. grouped vs xqa (clean reference) vs an einsum reference:

geometry	poison	poisoned dims	xqa NaN	grouped NaN (unfixed)	grouped NaN dims	max|grouped−xqa|
rows 19, contiguous, decode-like	targeted	[102,152,168,170,230]	0	480	[102,152,168,170,230]	1.5e-05
rows 24, 3-req, scatter, causal	targeted	[102,152,168,170,230]	0	640	[102,152,168,170,230]	2.4e-04
both	random page 0	random	0	matches poisoned dims	same	≤2.4e-04

With either fix (kernel, or Python pad-fix on the stock kernel) all geometries return grouped NaN = 0, bit-consistent across repeated runs, and equal to XQA to max|grouped−xqa| ≤ 2.4e-04. An unpoisoned regression stays NaN = 0 and matches the reference.

Microbenchmark (grouped route, 8192-token context, 16 rows, shared with other load so noisy): mean 0.187 → 0.233 ms/iter with the kernel fix (one extra __syncthreads() per tile); negligible in a real long-context prefill where the grouped kernel is a small fraction of the step.

End-to-end on 4× V100-PCIE-32GB TP4 with E4M3 KV + MTP4 (the configuration that exposed this): the previously-guaranteed-NaN count probe returns sane, finite logprobs; a 200K needle passes; a tail/K+1 probe passes with zero OOV; single-stream decode is ~97 tok/s at 8K and ~69 tok/s at 240K; outputs are byte-identical between the kernel fix and the Python pad-fix.

The E4M3 + MTP enablement that surfaced this path is separate work and is not part of this PR; this PR is only the null-block padding fix and its regression test.

Tests

tests/models/qwen4_exp/test_qsa_e4m3.py adds test_qsa_grouped_page4_null_block_padding_no_nan, an SM70-only regression (skips cleanly off SM70 or without the grouped/XQA page4 kernels). It builds a 32-token-page E4M3 cache whose page 0 is the null block, poisons its value plane with E4M3 NaN, and asserts the grouped route stays finite and matches XQA. It is parametrized over the Python pad-fix being off (kernel-only) and on, so it reproduces the NaN on an unfixed kernel and passes on either fix.

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