Re: [Qemu-devel] [libvirt] Modern CPU models cannot be used with libvirt

[Anthony Liguori <aliguori@us.ibm.com>]

On 03/11/2012 11:16 AM, Gleb Natapov wrote:
> On Sun, Mar 11, 2012 at 10:33:15AM -0500, Anthony Liguori wrote:
>> On 03/11/2012 09:56 AM, Gleb Natapov wrote:
>>> On Sun, Mar 11, 2012 at 09:12:58AM -0500, Anthony Liguori wrote:
>>>> -cpu best wouldn't solve this.  You need a read/write configuration
>>>> file where QEMU probes the available CPU and records it to be used
>>>> for the lifetime of the VM.
>>> That what I thought too, but this shouldn't be the case (Avi's idea).
>>> We need two things: 1) CPU model config should be per machine type.
>>> 2) QEMU should refuse to start if it cannot create cpu exactly as
>>> specified by model config.
>>
>> This would either mean:
>>
>> A. pc-1.1 uses -cpu best with a fixed mask for 1.1
>>
>> B. pc-1.1 hardcodes Westmere or some other family
>>
> This would mean neither A nor B. May be it wasn't clear but I didn't talk
> about -cpu best above. I am talking about any CPU model with fixed meaning
> (not host or best which are host cpu dependant). Lets take Nehalem for
> example (just to move from Westmere :)). Currently it has level=2. Eduardo
> wants to fix it to be 11, but old guests, installed with -cpu Nehalem,
> should see the same CPU exactly. How do you do it? Have different
> Nehalem definition for pc-1.0 (which level=2) and pc-1.1 (with level=11).
> Lets get back to Westmere. It actually has level=11, but that's only
> expose another problem. Kernel 3.3 and qemu-1.1 combo will support
> architectural PMU which is exposed in cpuid leaf 10. We do not want
> guests installed with -cpu Westmere and qemu-1.0 to see architectural
> PMU after upgrade. How do you do it? Have different Westmere definitions
> for pc-1.0 (does not report PMU) and pc-1.1 (reports PMU). What happens
> if you'll try to run qemu-1.1 -cpu Westmere on Kernel<  3.3 (without
> PMU support)? Qemu will fail to start.

So, you're essentially proposing that -cpu Westmere becomes a machine option and 
that we let the machines interpret it as they see fit?

So --machine pc-1.0,cpu=Westmere would result in something different than 
--machine pc-1.1,cpu=Westmere?

That's something pretty different than what we're doing today.  I think that we 
would have a single CPUX86 object and that part of the pc initialization process 
was to create an appropriately configured CPUx86 object.

Regards,

Anthony Liguori