Re: kern/59721: qemu VM's blackscreen over VNC wth -machine q35

["Taylor R Campbell via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR kern/59721; it has been noted by GNATS.

From: Taylor R Campbell <riastradh%NetBSD.org@localhost>
To: Roy Marples <roy%marples.name@localhost>,
	Michael van Elst <mlelstv%serpens.de@localhost>,
	Martin Husemann <martin%duskware.de@localhost>
Cc: gnats-bugs%NetBSD.org@localhost, netbsd-bugs%NetBSD.org@localhost
Subject: Re: kern/59721: qemu VM's blackscreen over VNC wth -machine q35
Date: Sun, 26 Oct 2025 23:09:13 +0000

 I found that nvmm is not actually the culprit here -- it is only
 indirectly responsible.  qemu's nvmm accelerator doesn't support SMM
 (System Management Mode), so
 
 -machine q35 -accel nvmm
 
 implies
 
 -machine q35,smm=off -accel nvmm
 
 and it turns out that `smm=off' is enough to reproduce the issue.
 That is, minimal reproducer (no extra arguments needed):
 
 (works) qemu-system-x86_64 -machine q35
 (fails) qemu-system-x86_64 -machine q35,smm=off
 
 I tried with qemu-6.2 and seabios-1.15.0 on Ubuntu, and both worked
 fine.  I also tried with qemu-10.1.1 and seabios-1.15.0 on NetBSD, and
 the bug reproduced.  So maybe something changed with smm=off between
 qemu-6.2 and qemu-10.1.1 that broke SeaBIOS but not OVMF UEFI.