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

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

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.