Re: [Qemu-devel] Target vs architecture for QEMU binary

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Sep 09, 2015 at 10:17:34AM +0200, Andrea Bolognani wrote:
> On Tue, 2015-09-08 at 16:47 +0100, Daniel P. Berrange wrote:
> > > Or we could just query everything that looks like a QEMU
> > > binary and then lookup the correct one for the guest based
> > > on the query results, couldn't we? Again, assuming such
> > > interface even exists.
> > 
> > I'd prefer libvirt to not have a trawl through every QEMU
> > binary to do this really.
> 
> AFAIK we're already querying every binary for other stuff
> we're interested in, so adding one more query shouldn't
> change anything. Or am I missing something?

In all the other cases we already know which binary we need
to query. We happen to cache the results of querying binaries
we find in $PATH, but none of our code has a fixed assumption
that we have caps available for every single binary. I don't
want such an assumption to get baked into libvirt code,
because it will limit our flexibility to change the way we
probe / cache capabilities data later, if we have a requirement
to always query every binary upfront.

> > > I'm not sure they're covering all possible combinations,
> > > though. Which is why it would be really nice to be able to
> > > ask this stuff to QEMU itself.
> > 
> > So, I think what we need do is to just refactor the
> > virQEMUCapsFindBinaryForArch(), to pull out the
> > architecture canonocalization out into a separate
> > method eg virArch virQEMUCapsCanonicalSystemArch(virArch)
> > and then just call it from both places
> 
> Sounds reasonable. Are we sure we have a complete
> understanding of the relationship between targets and
> architectures, though? For example, I don't see anything
> about s390, and the ARM stuff doesn't look like it covers
> everything. I just want to make sure we're not doing
> anything wrong or missing any possible combination.

Yeah, we've just added exceptions on an as-nedeed basis, so we could
well need more special cases added


Regards,
Daniel
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