using libvirt 4.5 with upstream qemu

[Daniel P. Berrangé]

On Mon, Mar 01, 2021 at 03:30:58PM +0000, Thanos Makatos wrote:
> I'm trying to use QEMU master with libvirt 4.5 and QEMU seems to be hanging
> when I try to start a guest.
> 
> My environment is a modified CentOS 7.9 installation using libvirt 4.5.0. When
> I use a modified version of QEMU 2.12 (reasonably close to the stock CentOS
> version) everything works fine. When I try to use a fairly recent version of
> QEMU (e.g. v5.2.0-729-g89ff714f4b).
> 
>   qemu     118657  1.6  0.0      0     0 ?        Z    14:50   0:00 [qemu-kvm] <defunct>
>   qemu     118664  0.0  0.0 207340  3560 ?        Ssl  14:50   0:00 /usr/libexec/qemu-kvm -S -no-user-config -nodefaults -nographic -machine none,accel=kvm:tcg -qmp unix:/var/lib/libvirt/qemu/capabilities.monitor.sock,server,nowait -pidfile /var/lib/libvirt/qemu/capabilities.pidfile -daemonize
>   qemu     118666  0.0  0.0 275008 13916 ?        Sl   14:50   0:00 /usr/libexec/qemu-kvm -S -no-user-config -nodefaults -nographic -machine none,accel=kvm:tcg -qmp unix:/var/lib/libvirt/qemu/capabilities.monitor.sock,server,nowait -pidfile /var/lib/libvirt/qemu/capabilities.pidfile -daemonize
> 
> /var/lib/libvirt/qemu/capabilities.pidfile contains the PID of the 2nd QEMU
> process and by experimenting I found that by killing the QEMU process NOT in
> the PID file the same thing happens again (maybe once more or twice) and then
> the guest boots fine.
> 
> Does libvirt have some specific QEMU dependency? Is there some compatibility
> matrix I might have missed? I understand this may not be a supported
> configuration given that I'm not using vanilla libvirt/QEMU, however I'd
> appreciate some pointers so I can further debug this. I've trying debugging
> this in case there's some obvious error but didn't find anything interesting.

You want libvirt to be the same age as the QEMU you're using, or newer.

New libvirt is expected to work with old QEMU.

Old libvirt may not work with new QEMU.

QEMU's deprecation process leads to code/behaviour being removed/changed
over time and this may break old libvirt. I don't know the cause of the
problem in your specific case, but personally i wouldn't spend time
trying to debug it as it isn't a scenario we attempt to support.

Regards,
Daniel
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