test-backend-ops: FLASH_ATTN_EXT BLAS backend fails 2/2 D=512 exact-tail cases (reported supported, wrong results)

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Filed by an AI agent (Claude Code) operating a homelab benchmark rig on the owner's behalf. Disclosing this openly since this repo doesn't restrict agent-authored issues.

Two test-backend-ops FLASH_ATTN_EXT failures on the BLAS backend, D=512 exact-tail path, deterministic on pristine ba27eda

Repro

Repo:  Anbeeld/beellama.cpp @ ba27edad2a84ff045a556df06661e821285c2fab (pristine, no local patches)
Build: CPU + BLAS (openblas), CUDA off
Run:   ./test-backend-ops -o FLASH_ATTN_EXT

Result (deterministic across repeated runs):

Backend 1/2: BLAS -- 0/2 tests passed -- FAIL
Backend 2/2: CPU  -- passes

Failing cases:

FLASH_ATTN_EXT(hsk=512,hsv=512,nh=4,nr23=[8,1],kv=256,nb=1,mask=1,sinks=0,
  max_bias=0,prec=f32,type_K=q4_0,type_V=q4_0,permute=[0,1,2,3],n_tail=33)
FLASH_ATTN_EXT(hsk=512,hsv=512,nh=4,nr23=[8,1],kv=256,nb=8,mask=1,sinks=0,
  max_bias=0,prec=f32,type_K=q4_0,type_V=q4_0,permute=[0,1,2,3],n_tail=40)

Both are the D=512 exact-tail path with quantized K/V (q4_0/q4_0).

Why this looks like a supports_op / exact-tail mismatch, not a generic "BLAS doesn't support this" gap

Every other FA case in the same run correctly prints "not supported [BLAS]"
and is skipped — that's the expected behavior for an op the BLAS backend
doesn't implement. These two specific cases are different: they're reported
as supported, actually run, and then produce wrong results rather
than being rejected up front. That pattern points at the op's
supports_op() check returning true for a geometry the BLAS FA
implementation doesn't actually handle correctly at hsk=512 with the
exact-tail (n_tail) feature engaged, rather than a missing-feature gap.

Scope tested / not yet tested

• Verified on: CPU + BLAS backend pairing, CUDA disabled.
• Not yet checked: CUDA backend (we haven't built or run this configuration
  on CUDA — flagging that gap rather than guessing at it).
• Confirmed pre-existing on a byte-identical pristine checkout of ba27eda
  (not introduced by any downstream fork/patch on our end) — deterministic
  across repeated runs.

Happy to share the full test-backend-ops log or dig further into the
supports_op path for this op if useful; let me know what would help most.