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

[Alexander Graf]

On 05.11.2013, at 17:16, Andreas Färber <address@hidden> wrote:

> Am 05.11.2013 07:05, schrieb Alexander Graf:
>> 
>> 
>> Am 05.11.2013 um 05:00 schrieb Paul Mackerras <address@hidden>:
>> 
>>> 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.
> 
> 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.

We could also just be nice and error out with a sane error message:

  Your selected CPU POWER7_v10 is not compatible with the host's CPU POWER7_20
  with your currently selected KVM mode. Please use the latter or -cpu host.


Alex