kern/60426: Signal handler corrupts AVX (YMM) registers

["bsiegert%netbsd.org@localhost via gnats" <gnats-admin%NetBSD.org@localhost>]

>Number:         60426
>Category:       kern
>Synopsis:       Signal handler corrupts AVX (YMM) registers
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jul 08 09:25:00 +0000 2026
>Originator:     Benny Siegert
>Release:        NetBSD 10.1_STABLE
>Organization:
The NetBSD Foundation
>Environment:
NetBSD 10.1/amd64
Architecture: x86_64
Machine: amd64
>Description:
This is a bug reported to Go about memory corruption in the runtime:

https://github.com/golang/go/issues/80285
runtime: async preemption corrupts AVX (YMM) state on netbsd/amd64

> It seems NetBSD 10.1/amd64 does not preserve the upper 128 bits of AVX (YMM)
> registers across signal delivery.

For now, the workaround has been to disable AVX completely in Go on NetBSD/amd64.
>How-To-Repeat:
> Repro (no Go, ymmsig.c attached to the bug report in GitHub): park a canary
> in ymm0, spin comparing it, while another thread sends SIGUSR1 whose handler
> clobbers YMM. On sigreturn the upper YMM half is not restored - the compare
> fails with mask=0000ffff (low XMM intact, high YMM lost). Clean on Linux
> (>1e9 iterations). Observed on QEMU -enable-kvm -cpu host (AMD Zen2 host,
> AVX2, no AVX-512), go-independent. This corrupts any AVX code interrupted by
> a signal (e.g. Go async preemption interrupting memmove).
>Fix:
Yes please!