pkg/59820: QEMU 10.1.2 no longer work with `-smp 2` (or more) with NVMM (qemu 10.1.1 -> 10.1.2 regression)

[Leonardo Taccari <leot%NetBSD.org@localhost>]

>Number:         59820
>Category:       pkg
>Synopsis:       QEMU 10.1.2 no longer work with `-smp 2` (or more) with NVMM (qemu 10.1.1 -> 10.1.2 regression)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Dec 06 17:50:00 +0000 2025
>Originator:     Leonardo Taccari
>Release:        NetBSD 11.99.3
>Organization:
The NetBSD Foundation
>Environment:
System: NetBSD abacus 11.99.3 NetBSD 11.99.3 (GENERIC) #0: Tue Oct 14 19:59:20 CEST 2025 leot@abacus:/usr/obj/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
>Description:
	QEMU 10.1.2 (qemu-10.1.2nb1) no longer works with `-smp 2` (or more)
	with NVMM. When trying to boot a virtual machine it completely hangs.

	This is a regression betwoon QEMU 10.1.1 (that worked) and QEMU 10.1.2
	(that get stuck).
>How-To-Repeat:
	Fetch Alpine Linux (I have not tested with other guests ATM):

	$ curl -OL "https://dl-cdn.alpinelinux.org/alpine/v3.22/releases/cloud/nocloud_alpine-3.22.0-x86_64-bios-cloudinit-r0.qcow2";

	Ensure that QEMU is installed and its version is 10.1.2:

	$ pkg_info | awk '/^qemu/ { print $1 }'
	qemu-10.1.2nb1

	Try to boot Alpine:

	$ qemu-system-x86_64 -version
	QEMU emulator version 10.1.2
	Copyright (c) 2003-2025 Fabrice Bellard and the QEMU Project developers
	$ qemu-system-x86_64 -m 1024 -cpu max -machine q35,accel=nvmm -smp 2 -boot order=c,splash-time=0 -drive file=nocloud_alpine-3.22.0-x86_64-bios-cloudinit-r0.qcow2,if=virtio,readonly=on -nographic
	NetBSD Virtual Machine Monitor accelerator is operational
	qemu-system-x86_64: NVMM: Unexpected WRMSR 0x3a [val=0x60005], ignored
	qemu-system-x86_64: NVMM: Unexpected WRMSR 0x3a [val=0x60005], ignored
	[... qemu get stucks and to terminate it, it needs a kill -9...]

	If `-smp 1` or no `-smp` option is provided it works.

	Please also note that downgrading to qemu-10.1.1.tgz works.
>Fix:
	No idea, sorry!

	Downgrading to qemu-10.1.1 or not using `-smp` option workarounds that.