qemu modularization of qemu-5.1 vs libvirt domcapabilities cache?

[Martin Wilck]

On Thu, 2020-08-20 at 10:57 +0100, Daniel P. Berrangé wrote:
> On Thu, Aug 20, 2020 at 11:32:03AM +0200, Martin Wilck wrote:
> > On Tue, 2020-08-18 at 15:15 -0600, Jim Fehlig wrote:
> > > On 8/5/20 2:19 AM, Andrea Bolognani wrote:
> > > > 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.
> > > 
> > > We recently received reports of this issue on Tumbleweed, which
> > > just
> > > got the 
> > > modularized qemu 5.1
> > > 
> > > https://bugzilla.opensuse.org/show_bug.cgi?id=1175320
> > > 
> > > Mark, are you working on a patch to invalidate the cache on
> > > changes
> > > to the qemu 
> > > modules directory? I suppose it needs to be handled similar to
> > > the
> > > qemu 
> > > binaries. E.g. when building the cache include a list of all qemu
> > > modules found. 
> > > When validating the cache see if any modules have disappeared, if
> > > any
> > > have been 
> > > added, and if the ctime of any have changed. Yikes, sounds a
> > > little
> > > more complex 
> > > than the binaries :-).
> > 
> > I'd like to question whether this effort is justified for an
> > optimization that matters only at libvirtd startup time, and even
> > there
> > saves no more than a few seconds.
> > 
> > I propose to simply disable the caching of qemu capabilities (or
> > provide a build-time option to do so). Optimizations that produce
> > wrong
> > results should be avoided.
> 
> Whether the time matters depends on your use case for QEMU. For heavy
> data center apps like OpenStack you won't notice it because OpenStack
> itself adds soo much overhead to the system.  For cases where the VM
> is used as "embedded" infrastructure startup time can be critical.
> Not caching capabilities adds easily 300-500 ms to startup of a
> single
> VM, which is very significant when the current minimum startup time
> of
> a VM can be as low as 150 ms.
> 
> IOW removing caching is not a viable option.

Capability caching could be turned into a build-time option, optimized
for the target audience.

Or we could enable caching in general, but always rescan capabilites at
libvirtd startup. That way startup of VMs wouldn't be slowed down. No?

The latter would have the additional advantage that people who
(de)install qemu modules would simply need to restart libvirtd in order
to get a working system back. I suppose that would make sense to most
users, and probably even occur to them as a workaround. Currently users
have to locate and manually delete the cache files, not to mention that
they need to figure this out first, which is far from easy.

Regards
Martin