[libvirt] [RFC] Can we error out early for unknown device models?

[Daniel P. Berrange]

On Fri, Oct 24, 2014 at 12:15:24PM +0200, Martin Kletzander wrote:
> On Fri, Oct 24, 2014 at 07:16:44AM +0200, Michal Privoznik wrote:
> >On 22.10.2014 13:58, Martin Kletzander wrote:
> >>Hi everyone,
> >>
> >>I had this idea that since we are probing QEMU binaries for devices
> >>using 'qom-list-types', we could store that data in the capabilities
> >>and check whether device models are supported before starting QEMU.
> >>We do that for _some newer_ devices, but this would be global.  It
> >>would help out particularly with devices like the following one, for
> >>example:
> >>
> >><interface type='network'>
> >>   <source network='default'/>
> >>   <model type='non-existing_device'/>
> >></interface>
> >>
> >>where we simply construct QEMU command-line with that device model and
> >>it then properly errors out:
> >>
> >>error: internal error: process exited while connecting to monitor:
> >>qemu-system-x86_64: -device non-existing_device,...: Parameter
> >>'driver' expects device type
> >>
> >>This would be easy to achieve with current data unless there are some
> >>models hidden from qom-list-types' output.  I hope there are none.
> >
> >Well, this may vary among distros. While one distro lives with bleeding
> >edge, the other may just backport some patches. Even those adding new
> >devices. So I wouldn't rely on that.
> >
> 
> I don't quite get what you mean.  We'd be still compatible with old
> QEMU binaries that don't provide the data.
> 
> >>
> >>When I checked the output of 'qemu-kvm -device \?', the devices listed
> >>there are separated into categories and have buses assigned to them
> >>and that lead me to another idea.  What if that data is added to
> >>qom-list-types' output and used in libvirt as well?  We could then
> >>solve another annoying cases like misusing devices or plugging them
> >>into unsupported buses.  Although I don't know any person who would do
> >>such a thing, even when solving the first idea, there'd still be a
> >>possibility to do a thing like:
> >>
> >><interface type='network'>
> >>   <source network='default'/>
> >>   <model type='AC97'/>
> >></interface>
> >>
> >>This idea is obviously meant for the QEMU driver, but if there's
> >>something similar in other drivers, it might be useful as well.
> >>
> >>I'd be interested in any feedback and welcome any ideas to whether it
> >>is useful, how the storing should be done or even if it's something we
> >>want to have or not.
> >
> >So do you suggest that the check should be done on domain startup or
> >domain define time? It can't be the latter - libvirt should expose all
> >the domains it knows of - even these that can't be started. The idea
> >was, users can downgrade qemu and libvirt should cope with that.
> >If you, however, intend to do this at runtime, I see this as a gap in
> >our capabilites querying/checking. Or am I missing something here?
> >
> 
> Wherever the capabilities are checked now.  Of course you cannot do
> this when defining the domain.  This would be just an additional check
> when building the command line.  And the gap is what I'm suggesting to
> fix.  Just to be clear, the only output of such patches would be a
> nicer error message without relying on starting QEMU, nothing more.

If we can get better error reporting of devices that QEMU does not
support, when we start QEMU that is a win for me.  QEMU has a really
awful error message for devices that don't exist.


Regards,
Daniel
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