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

[Daniel P. Berrange]

On Tue, Sep 08, 2015 at 03:27:38PM +0200, Andrea Bolognani wrote:
> Hi,
> 
> 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.

> 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.

> 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

Regards,
Daniel
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