qemu modularization of qemu-5.1 vs libvirt domcapabilities cache?

[Daniel P. Berrangé]

On Wed, Aug 05, 2020 at 10:19:26AM +0200, Andrea Bolognani wrote:
> On Wed, 2020-08-05 at 02:09 -0400, Mark Mielke wrote:
> > Hi all:
> > 
> > In testing qemu-5.1rc2 on my Fedora 32 home system, I found that
> > the Fedora rawhide package has broken out both the QXL display
> > device and the USB redirect device into separate RPM modules:
> > 
> > qemu-device-display-qxl.x86_64         2:5.1.0-0.1.rc2.fc32
> > qemu-device-usb-redirect.x86_64        2:5.1.0-0.1.rc2.fc32
> > 
> > The upgrade from 5.0.0 to 5.1.0 does not treat these sub-packages
> > as mandatory packages, therefore a straight upgrade of packages
> > causes these domcapabilities to disappear.
> > 
> > If the user tries to use libvirt after this, then existing domains
> > using QXL display device or USB redirect device will fail to start.
> > If the user then investigates and realizes that they now need to
> > install the above packages separately, they find that qemu-kvm
> > recognizes the modules right away, but libvirt does not. This looks
> > to be due to the libvirt domcapabilities cache?
> 
> I guess we need to start checking the modules directory in addition
> to the main QEMU binary, and regenerate capabilities every time its
> contents change.
> 
> That said, if upgrading QEMU results in losing features, even though
> you can recover them through additional steps I would argue that's a
> bug in the packaging that should be addressed on the QEMU side.

Potentially we can consider this a  distro packaging problem and fix
it at the RPM level.

eg the libvirt RPM can define a trigger that runs when *any* of the
qemu-device*  RPMs is installed/updated. This trigger can simply
touch a file on disk somewhere, and libvirtd can monitor this one
file, instead of having to monitor every module.


Regards,
Daniel
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