Re: [Qemu-devel] [PATCH] RFC: powerpc: add PVR compatibility check

[Paul Mackerras <paulus@samba.org>]

On Tue, Nov 05, 2013 at 05:16:33PM +0100, Andreas Färber wrote:
> Am 05.11.2013 07:05, schrieb Alexander Graf:
> > 
> > 
> > Am 05.11.2013 um 05:00 schrieb Paul Mackerras <paulus@samba.org>:
> > 
> >> On Mon, Nov 04, 2013 at 10:05:58AM +0100, Alexander Graf wrote:
> >>>
> >>> Yeah, we really need to check that guest vpcu == host vcpu for HV KVM.
> >>
> >> In general I agree, but the one difficulty I see is that a check for
> >> exact equality will interact badly with qemu's habit of picking a
> >> specific processor version when the user specifies something general
> >> like "POWER7".  So if the user does -cpu POWER7 on a machine with
> >> (for example) a POWER7 v2.1 processor, but qemu arbitrarily picks the
> >> PVR for POWER7 v2.3, then it will fail, which will be completely
> >> puzzling to the user -- "I asked for POWER7, and it is a POWER7,
> >> what's the problem??".
> >>
> >> Maybe if the user asks for a non-specific processor type, and the
> >> host's PVR matches the request, then qemu should take the host's PVR
> >> rather than just picking some arbitrary processor version.
> > 
> > Yup.
> 
> But then it's no longer generally reproducible: "POWER7" won't be
> "POWER7" on another machine.

There aren't any observable differences between POWER7 versions that
have been sold to customers, as far as I have been able to ascertain
(other than the PVR value, of course).  So this whole business of
carefully distinguishing between POWER7 v2.2 and POWER7 v2.3 is
largely a waste of time as far as I can see.

I admit that in the past we (IBM) did a silly thing in releasing the
POWER5+ v3.0 chip with some architecturally new features (64k pages
and some other MMU changes).  That was a mistake and I don't think
we'll do it again.

I think the default assumption should be that versions of a given IBM
POWER chip (identified by the upper 16 bits of the PVR) are
architecturally identical, and behaviourally identical at the level
at which QEMU models the chip.  Differences between chips would
normally be limited to bug fixes and performance improvements.  Then
we just need a way to cope with POWER5+ v3.0.

> One thing I original did iirc was to hide the aliases from QMP. You can
> always do stupid things on the command line and then we can blame you,
> but if libvirt and upper layers don't offer "POWER7" to the end user
> then we don't need to worry about the average user misinterpreting its
> semantics.

Given that the only difference between POWER7 v2.2 and POWER7 v2.3
(say) will be which set of host systems you get an error on, there
doesn't seem to me to be a lot of point.

Paul.