No virtio devices in SeaBIOS VMs

[Dario Faggioli]

Hello everyone,

With QEMU 8.2, guests that:
 - use SeaBIOS
 - use something different than "-cpu host" OR don't use "host-phys-
bits=on"
 - have more than 2815 MB of RAM

have problems with their virtio devices and, hence, malfunction in
various ways (e.g., if they're using a virtio disk, they don't find it,
and the VM does not boot).

This broke all of a sudden, as soon as we updated QEMU to 8.2.0 in
openSUSE, and we got some bugreports about it. E.g., this one [1]
includes some logs and info, but I can provide more, if helpful.

I did try master (instead of 8.2, although it was master as of last
week), but the problem was still there.

I've then bisected it to SeaBIOS commit:
96a8d130a8c2e908e357ce62cd713f2cc0b0a2eb ("be less conservative with
the 64bit pci io window"). In fact, falling back to an earlier SeaBIOS
version, before that commit, or even just reverting it [1], solves the
issue.

UEFI guests seem not to be affected in any way, no matter amount of RAM
or CPU model (well, of course, since it's a SeaBIOS commit! :-D What I
mean is that there seems to be nothing in edk2 that induces the same
behavior).

A way of working this around (beside switching to UEFI or to cpu=host)
is to turn on host-phys-bits, e.g., with '<maxphysaddr
mode="passthrough"/>' in the XML.

It is, however, a bit impractical to have to do this for all the VMs
that one may have... Especially if they're a lot! :-)

I know that there have been issues and discussions (and they were also
related to virtio, I think) about these changes already. I don't know
if it's the same or a related problem but is there a way to avoid
having to ask people to change all their VMs' config?

Thanks and Regards,
Dario

[1] https://bugzilla.suse.com/show_bug.cgi?id=1219977
[2] well, I actually reverted a6ed6b701f0a57db0569ab98b0661c12a6ec3ff8
too, for convenience
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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