s390x: 4x8 GEMM micro-kernel, measured on a real IBM z17; Go 1.27.2

[tannevaled]

s390x on real hardware (IBM z17)

The LinuxONE VM now answers at a new address (ssh linuxone). Verified in /proc/sysinfo: IBM z17 (Type 9175), a z/VM 7.3.0 guest with 8 vCPUs in a shared LPAR. v0.8.0's s390x kernels, until now validated under emulation only, now run there:

• The whole suite passes, as do the fence tests (sumVX reading len+1 faults on 4 KiB pages) and TestParallelStress (27,752 calls).
• New 4×8 GEMM micro-kernel gemmMicro4x8VX: 16 two-lane accumulators, the B row in one VLM, A replicated with VLREPG, fused with VFMADB. Sabotage caught on the z17: dropping tile row 3 gives MatMul m=50 k=70 n=90 [270]: 0 != -249.

Measured with hostload (0% busy, 0% steal) + benchstat, 8 interleaved runs each, p = 0.000 unless marked:

z17, time	1 core	8 cores
Sum	−85…−87%	−82…−87%
AddInto/MulInto	−44…−61%	−36…−60%
MatVec	−44%	−40%
Dot	−11%	~
MatMul 1024² (this PR)	340 → 70 ms	43 → 10 ms
GEMM geomean (11 shapes)	−74%	−66%

Exp is untouched by any of this and serves as the control: ±0.3%.

Go floor 1.27.2

• Floor raised. At 1.27.1, govulncheck lists stdlib vulns fixed in 1.27.2 (GO-2026-6604/6612/6613/6617…), none reachable. At 1.27.2 it reports none. go.mod, the generator go.mod, every CI lane and the README badge are updated.
• Correction. SECURITY.md had said cmd/compile: loong64 spills a byte-sized inlined result with an 8-byte MOVD, clobbering adjacent stack slots golang/go#81000 (loong64 miscompile) was fixed in 1.27.2. That is wrong: the backport (#81147) is on the 1.27.3 milestone, and the upstream reproducer still fails with go1.27.2 on cfarm401 (real Loongson). The old entry is struck through and annotated; the floor goes to 1.27.3 when it ships.

Docs

• perf.md s390x section: emulation-only → measured on the z17, with the table above.
• README: s390x 4×8 in the GEMM list, the "emulation only" sentence removed, plus the playground badge and line requested by the go-fft coordinator. https://go-ndarray.github.io/playground/ returns 200.
• Plan updated, and the SECURITY.md audit entry added.

Version: v0.9.0 (Go floor raise + new kernel).

🤖 Generated with Claude Code