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

[Andrea Bolognani]

On Tue, 2015-09-08 at 15:37 +0100, Daniel P. Berrange wrote:
> > at the moment, libvirt is using some ad-hoc logic to allow
> > i686 guests to run on qemu-system-x86_64 (by using the CPU
> > model qemu32); in all other cases, it's assumed that a $arch
> > guest needs qemu-system-$arch to run.
> > 
> > This is causing a problem right now with ppc64le guests
> > because, even though qemu-system-ppc64 is perfectly capable
> > of running them, libvirt will refuse to.
> 
> Is there a bug report somewhere for that, because libvirt
> already has code in virQEMUCapsFindBinaryForArch() which
> forces it to look at qemu-system-ppc64 when asked to use
> ppc64le, so I'd expect it to already work.

You're right, starting a guest actually works.

The bug report[1] (which was filed by none other than myself :)
is still related to figuring out stuff starting from the guest
architecture, but it's apparently going through a different
code path where the adjustment you're referring to is not
applied[2].

> > We want to change the logic so that it reflects the actual
> > capabilities of the QEMU binary, but AFAICT there isn't eg.
> > a QMP command we can use to query the binary for the list
> > of architectures it implements.
> > 
> > Am I missing something? Is such an interface available?
> 
> We have a bit of a chicken and egg problem, because to query
> QEMU for capabilities, you already have to know what system
> emulator binary is required for the architeture you want to
> run.

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.

> > Failing that, we'll have to map QEMU targets with implemented
> > guest architectures inside libvirt, in which case it would be
> > great if you could point me towards either some up-to-date
> > documentation or a reliable way to extract the information
> > myself.
> 
> The various rules in virQEMUCapsFindBinaryForArch() already
> try todo a suitable mapping

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.

Cheers.


[1] https://bugzilla.redhat.com/show_bug.cgi?id=1260753
[2] If I'm not mistaken, qemuCaps->arch is set in
    virQEMUCapsInitArchQMPBasic() and none of the logic
    you're talking about seems to be used there.
-- 
Andrea Bolognani
Software Engineer - Virtualization Team