[libvirt] [PATCH v2 06/15] qemu: Defer capability check to command line generation time

[Daniel P. Berrangé]

On Mon, Feb 19, 2018 at 10:29:18AM +0100, Andrea Bolognani wrote:
> On Mon, 2018-02-19 at 07:24 +0100, Peter Krempa wrote:
> > On Fri, Feb 16, 2018 at 17:28:03 +0100, Andrea Bolognani wrote:
> > > Validate time is a bit too early to check whether the required
> > > capabilities are available, since the QEMU binary might have
> > > been updated or replaced by the time we are asked to run the
> > > guest.
> > 
> > So are you having problem with the fact that the definition will be
> > rejected right away and not just when you try to start it?
> > 
> > Validate is re-run when starting the VM so a downgrade is handled
> > properly.
> 
> Right, but isn't checking for QEMU capabilities at validate time
> unreasonably strict? A guest which uses eg. an invalid combination
> of machine type and architecture will never become valid at a later
> point, but a guest should not be considered invalid just because
> the QEMU binary you happened to have installed at the time you
> defined it lacked some features - the guest itself is perfectly
> valid, it just can't be run :)

Given that we increasingly fill in alot of information in the XML at define
time, we already have a general expectation that the QEMU binary will  be
present at define time. I think this is not unreasonable - we suggest apps
call virConnectGetCapabilities and/or virDomainGetCapabilities to understand
what is installed/available before creating an XML document to define. Those
APIs of course require binaries to be installed too.   So I don't think we
should really put effort into coping with defining XML for a time when the
QEMU binaries aren't installed.  Such a scenario is so unlikely to be hit
that any code trying to cope with that is going to be largely untested and
fragile, so it would be a disservice to pretend it'll be something worth
relying on.

Regards,
Daniel
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