libvirt users - Oct 2018 - How to explain this libvirt oddity w.r.t machine types?

Context: The baremetal host previously had QEMU 2.11.  But I manually
downgraded the QEMU version (via `dnf downgrade qemu-system-x86`); now
it is at 2.10:

    $ rpm -q qemu-system-x86
    qemu-system-x86-2.10.2-1.fc27.x86_64

The guest is offline.  Let's see (in a couple of ways) what machine type 
it has while it is dormant:

    # virsh dumpxml cirros | grep -i machine        <type
arch='x86_64' machine='pc-i440fx-2.10'>hvm</type>

    # grep machine= /etc/libvirt/qemu/cirros.xml
        <type arch='x86_64'
machine='pc-i440fx-2.10'>hvm</type>

Okay, now edit the guest XML and carefully remove the
"machine='pc-i440fx-2.10'" bit---to see what machine type will
libvirt
(libvirt-daemon-kvm-4.0.0-2.fc27.x86_64) default to:

    # virsh edit cirros
    Domain cirros XML configuration edited.
    
Now check the machine type again.  Bizarrely enough, libvirt
"helpfully"
auto-adds QEMU *2.11* machine type, which is obviously no longer on the
system!

    # grep machine= /etc/libvirt/qemu/cirros.xml
    <type arch='x86_64'
machine='pc-i440fx-2.11'>hvm</type>

    # virsh dumpxml cirros | grep -i machine    <type arch='x86_64'
machine='pc-i440fx-2.11'>hvm</type>

How to explain this?  Is this even a "valid test"?

(To undo the nuisance, obviously, I had to `virsh edit cirros` again and
change it to 2.10.)

Note, I *don't* have 2.11 QEMU on the system:

    # rpm -qa | grep -E 'qemu.*2.11'
    # echo $?
    1

        * * *

It's getting late, and I should stop staring at screens.

-- 
/kashyap

On Fri, 2018-10-12 at 22:38 +0200, Kashyap Chamarthy
wrote:
> Context: The baremetal host previously had QEMU 2.11.  But I manually
> downgraded the QEMU version (via `dnf downgrade qemu-system-x86`); now
> it is at 2.10:
> 
>     $ rpm -q qemu-system-x86
>     qemu-system-x86-2.10.2-1.fc27.x86_64
> 
> The guest is offline.  Let's see (in a couple of ways) what machine
type
> it has while it is dormant:
> 
>     # virsh dumpxml cirros | grep -i machine>         <type
arch='x86_64' machine='pc-i440fx-2.10'>hvm</type>
> 
>     # grep machine= /etc/libvirt/qemu/cirros.xml
>         <type arch='x86_64'
machine='pc-i440fx-2.10'>hvm</type>
> 
> Okay, now edit the guest XML and carefully remove the
> "machine='pc-i440fx-2.10'" bit---to see what machine type
will libvirt
> (libvirt-daemon-kvm-4.0.0-2.fc27.x86_64) default to:
> 
>     # virsh edit cirros
>     Domain cirros XML configuration edited.
>     
> Now check the machine type again.  Bizarrely enough, libvirt
"helpfully"
> auto-adds QEMU *2.11* machine type, which is obviously no longer on the
> system!
> 
>     # grep machine= /etc/libvirt/qemu/cirros.xml
>     <type arch='x86_64'
machine='pc-i440fx-2.11'>hvm</type>
> 
>     # virsh dumpxml cirros | grep -i machine>     <type
arch='x86_64' machine='pc-i440fx-2.11'>hvm</type>
> 
> How to explain this?  Is this even a "valid test"?
> 
> (To undo the nuisance, obviously, I had to `virsh edit cirros` again and
> change it to 2.10.)
> 
> Note, I *don't* have 2.11 QEMU on the system:
> 
>     # rpm -qa | grep -E 'qemu.*2.11'
>     # echo $?
>     1
> 
>         * * *
> 
> It's getting late, and I should stop staring at screens.
If I had to guess, I would say the <emulator> element of your guest
is probably pointing to a custom-built QEMU 2.11 binary rather than
the default one installed from RPMs.

-- 
Andrea Bolognani / Red Hat / Virtualization

On Sun, Oct 14, 2018 at 02:28:07PM +0200, Andrea Bolognani
wrote:
> On Fri, 2018-10-12 at 22:38 +0200, Kashyap Chamarthy wrote:
[...]
> > Now check the machine type again.  Bizarrely enough, libvirt
"helpfully"
> > auto-adds QEMU *2.11* machine type, which is obviously no longer on
the
> > system!
> > 
> >     # grep machine= /etc/libvirt/qemu/cirros.xml
> >     <type arch='x86_64'
machine='pc-i440fx-2.11'>hvm</type>
> > 
> >     # virsh dumpxml cirros | grep -i machine> >     <type
arch='x86_64' machine='pc-i440fx-2.11'>hvm</type>
> > 
> > How to explain this?  Is this even a "valid test"?
> > 
> > (To undo the nuisance, obviously, I had to `virsh edit cirros` again
and
> > change it to 2.10.)
> > 
> > Note, I *don't* have 2.11 QEMU on the system:
[...]
> If I had to guess, I would say the <emulator> element of your guest
> is probably pointing to a custom-built QEMU 2.11 binary rather than
> the default one installed from RPMs.
Your guess made me curious enough to check the remote machine one last
time before I went to sleep, but unfortunately the <emulator> element is
pointing to the plain `/usr/bin/qemu-kvm`, which in turn exec()'s
`/usr/bin/qemu-system-x86_64`.

(I attached the nothing-special, full guest XML.)


-- 
/kashyap