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Re: [libvirt] [PATCH] cpu_ppc64: Add support for host-model on POWER9



On Thu, Jun 08, 2017 at 10:21:44AM +0200, Andrea Bolognani wrote:
> On Thu, 2017-06-08 at 14:12 +1000, David Gibson wrote:
> > > David, are these known issues in QEMU? Are they being tracked?
> > 
> > It's not an issue in qemu, it's an issue in the hardware.
> > 
> > The various bugs in DD1 silicon mean it really isn't compliant with
> > the 3.00 architecture (nor, quite, with the earlier ones).  For that
> > reason we explicitly disable compatibility modes on a DD1 host.
> > 
> > More specifically advertising architected mode compatibility causes
> > the guest kernel not to turn on some vital DD1 workarounds, which
> > makes it break horribly.
> 
> Fair enough. That still doesn't account for the lack of
> power9 compatibility mode, which AFAICT is simply not
> defined at all in QEMU.

Umm.. what?  It's right there in target/ppc/compat.c.  Or are you
looking at downstream - maybe it hasn't been backported yet.

> Are there plans to introduce it? Should I file a bug to
> track it?
> 
> > > I don't see much point in updating libvirt to cope with these
> > > new compat modes when QEMU itself doesn't.
> > 
> > Try TCG, it should work there.
> 
> Not quite:
> 
>   $ /usr/libexec/qemu-kvm \
>     -nodefaults \
>     -M pseries,accel=tcg \
>     -cpu host,compat=power8
>   qemu-kvm: Unable to find CPU definition: host

Oh, right, of course, the "host" cpu only exists with KVM.

> I *can* make it work with
> 
>   $ /usr/libexec/qemu-kvm \
>     -nodefaults \
>     -M pseries,accel=tcg \
>     -cpu POWER9,compat=power8
>   VNC server running on ::1:5900
> 
> but that's not helpful because <cpu mode='host-model'/>
> uses the former. The same was true with POWER8, though.
> 
> 
> One more issue I've noticed:
> 
>   $ /usr/libexec/qemu-kvm \
>     -nodefaults \
>     -M pseries,accel=kvm \
>     -cpu POWER9
>   qemu-kvm: Unable to find sPAPR CPU Core definition
> 
> Since the recommended configuration for POWER8 guests is
> to use
> 
>   <cpu mode='custom'>
>     <model>POWER8</model>
>   </cpu>
> 
> I would expect the above command line to work so that
> POWER9 guests can be configured in a similar manner. That
> seems bug-worthy to me, should I go ahead and file it?

Uh, yeah.  Again, I'm pretty sure that's already working upstream.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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