[PATCH 4/5] qemu: Don't cache domCaps in virQEMUDriverGetDomainCapabilities()

[Cole Robinson]

On 11/16/20 2:43 AM, Michal Privoznik wrote:
> Currently, whenever a domain capabilities is needed (fortunately,
> after cleanup done by previous commits it is now only in
> virConnectGetDomainCapabilities()), the object is stored in a
> cache. But there is no invalidation mechanism for the cache
> (except the implicit one - the cache is part of qemuCaps and thus
> share its lifetime, but that is not enough). Therefore, if
> something changes - for instance new firmware files are
> installed, or old are removed these changes are not reflected in
> the virConnectGetDomainCapabilities() output.
> 
> Originally, the caching was there because domCaps were used
> during device XML validation and they were used a lot from our
> test suite. But this is no longer the case. And therefore, we
> don't need the cache and can construct fresh domCaps on each
> virConnectGetDomainCapabilities() call.
> 

Maybe the caching is not needed or was never needed. I added it as a
premature optimization to facilitate sharing validation data between
qemu and domcaps code. There's more info here:

https://www.redhat.com/archives/libvir-list/2019-April/msg00450.html

I was trying to establish a pattern where there was only one source of
truth for things like 'this qemu supports VGA video'. The linked mail
spells it out.

My fear is that if domcapabilities data is duplicating what is done at
qemu validation time, it's much more likely that domcapabilities content
will bitrot. It's not hard to imagine a scenario where qemu validation
gets tighter or more nuanced based on changing qemu capabilities, but
domcapabilities data is not updated to match, and now domcapabilities is
advertising a feature that qemu_validation.c immediately rejects. If
that feature is say a video model that an app like virt-install or
virt-manager is programmatically setting by default, now those tools are
generating invalid configurations that libvirt claims qemu supports.
Inaccurate domcapabilities can be worse than if nothing was reported at all.

- Cole