Re: qemu guest agent with openbsd

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On Fri, May 10, 2024 at 11:01:06AM GMT, Christian Haumesser via Users wrote:
> Hello,
> I have an OpenBSD 7.5 guest running on Debian bookworm with libvirt
> (9.0) and qemu (7.2).
> I'd like to use the qemu guest agent in this guest, but I can't
> seem to figure out how to craft the libvirt xml to expose the
> serial port in a way that OpenBSD and libvirt can use it together
> (or if it’s even possible).
> Per the libvirt docs
> <>, I’m currently
> using:
> > <channel type='unix'>
> >    <source mode='bind'/>
> >    <target type='virtio' name='org.qemu.guest_agent.0'/>
> > </channel>
> But OpenBSD doesn’t directly support the virtio console driver.
> Consistent with this 2020 thread
> <;sid=20200514073852> from
> the OpenBSD ports making list, I see the following message in my
> guest’s dmesg output:
> > virtio5 at pci0 dev 10 function 0 "Qumranet Virtio Console" rev 0x00
> > virtio5: no matching child driver; not configured
> Thanks to this libvir-list thread
> <>
> from 2015, I’ve realized that I can manually expose an ISA serial
> console that will allow me to connect to the guest agent using
> native qemu tooling; but there doesn’t seem to be a way to create a
> channel that libvirt can communicate over without virtio console
> support in the guest. The thread discusses a couple of approaches
> to resolving this issue with code changes, but it seems like the
> discussion stalled out before anything happened.
> Am I missing something or is it still impossible?

I just tried on both FreeBSD 14.0 and OpenBSD 7.5. It works out of
the box on the former, doesn't seem to work at all on the latter.

What I find weird is that a driver seems to exist already:

A few limitations are listed, and I'm not sure whether any of those
are relevant for this scenario. But I've also tried to set up a
virtio-console (rather than virtio-serial) and I couldn't get that to
work either, despite it supposedly being the whole purpose of the
driver. I'm not very familiar with OpenBSD though, so it's entirely
possible that I've simply made a mistake somewhere :)

In any case, my opinion is that the solution isn't changing libvirt
so that exposing multiple isa-serial devices becomes possible, but
rather enhancing the OpenBSD driver so that it supports virtio-serial
channels, the same way that other operating systems can. The FreeBSD
driver can likely be used as inspiration.

Andrea Bolognani / Red Hat / Virtualization
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