Re: [Qemu-devel] [PATCH] target-ppc: Add POWER8E_v2.1 CPU model.

[Andrea Bolognani]

On Wed, 2015-07-08 at 17:02 +1000, Alexey Kardashevskiy wrote:
> 
> > > > I think it's because libvirt likes to specify a specific CPU -
> > > > and if
> > > > it gets the new PVR from the host, qemu won't understand it.
> > > 
> > > A specific CPU in this case is "POWER8", I added this
> > > specifically for
> > > libvirt (to allow migration between all versions of POWER8), it
> > > should not
> > > use versioned CPUs and it does not in powerkvm.

Does "all versions of POWER8" include things like POWER8E, POWER8NVL
and "POWER8 DD1", as one of the variations is known in the kernel
source? Can we safely migrate guests eg. from a POWER8 v1.0 host to
a POWER8E v2.1 host?

> > Uh.. won't that make qemu attempt to set a specific PVR, though -
> > which will fail with recent KVM if it's not *exactly* the same as
> > the
> > host PVR.
> 
> The "POWER8" CPU class is created dynamically (like the "host" CPU
> class)
> and has the actual host PVR so setting it to KVM cannot fail.
> 
> When TCG, the class is not registered and alias is used instead.

Can you please point out where the dynamical creation happens for the
POWER8 class? I haven't been able to locate it, but then again I'm
all but familiar with QEMU's source :)

>From libvirt's point of view, it would be nice to be able to identify
as "POWER8" anything that looks like it, by matching the host's PVR
agains a numer of known PVRs (with relative bitmask). Ideally, if the
host can be identified as POWER8, that's the only CPU model libvirt
should advertise...

Cheers.

-- 
Andrea Bolognani
Software Engineer - Virtualization Team