-----Original Message----- From: Daniel P. Berrangé <berrange(a)redhat.com&gt; Sent: 01 March 2021 15:39 To: Thanos Makatos <thanos.makatos(a)nutanix.com&gt; Cc: libvir-list(a)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.

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.

On Mon, Mar 01, 2021 at 15:30:58 +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 These are possibly a leftover from libvirt's capability probing. I recall that we had some issue with capability detection qemu instances getting stuck but I don't remember the details any more.

> /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. > > Any ideas? In general, libvirt can't guarantee support of future qemu versions as qemu evolving and recently also deprecating and removing superseded configuration approaches, so you must cross-check with qemu's support policy. For debugging, debug logs may be helpful. In case a real VM getting stuck also the VM log file, but in your case the capability processes don't have that.

> -----Original Message----- > From: Daniel Henrique Barboza <danielhb413(a)gmail.com&gt; > Sent: 01 March 2021 18:07 > To: Peter Krempa <pkrempa(a)redhat.com>; Thanos Makatos > <thanos.makatos(a)nutanix.com&gt; > Cc: libvir-list(a)redhat.com > Subject: Re: using libvirt 4.5 with upstream qemu > > > > On 3/1/21 12:47 PM, Peter Krempa wrote: >> On Mon, Mar 01, 2021 at 15:30:58 +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 >> >> These are possibly a leftover from libvirt's capability probing. I >> recall that we had some issue with capability detection qemu instances >> getting stuck but I don't remember the details any more. > > Not sure if related to this reported problem, but upstream QEMU when > compiled with the trace backend was having problems daemonizing. I > experienced a similar issue in the start of the year when running upstream > Libvirt with upstream QEMU. > > I posted a patch fixing it in QEMU [1] but it wasn't pushed as of today. > I am not sure if the problem was fixed in another way or if the patch I posted > ended up left behind. > > Thanos, if you're compiling QEMU with a trace backend (e.g. with --enable- > trace-backend= in ../configure) it might be worth compiling it without this > option. There is a chance you're hitting same problem I mentioned above. Thanks Daniel, removing --enable-trace-backend did fix the issue!