using libvirt 4.5 with upstream qemu

[Thanos Makatos]

> -----Original Message-----
> From: Daniel P. Berrangé <berrange at redhat.com>
> Sent: 01 March 2021 15:39
> To: Thanos Makatos <thanos.makatos at nutanix.com>
> Cc: libvir-list at redhat.com
> Subject: Re: using libvirt 4.5 with upstream qemu
> 
> 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.

Makes sense, thanks.

> 
> 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.