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

[Gleb Natapov <gleb@redhat.com>]

On Mon, Mar 12, 2012 at 06:41:06PM +0100, Andreas Färber wrote:
> Am 12.03.2012 17:50, schrieb Eduardo Habkost:
> > On Mon, Mar 12, 2012 at 04:49:47PM +0100, Andreas Färber wrote:
> >> Am 11.03.2012 17:16, schrieb Gleb Natapov:
> >>> 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.
> [...]
> >> IMO interpreting an explicit -cpu parameter depending on -M would be
> >> wrong. Changing the default CPU based on -M is fine with me. For an
> >> explicit argument we would need Westmere-1.0 analog to pc-1.0. Then the
> >> user gets what the user asks for, without unexpected magic.
> > 
> > It is not unexpected magic. It would be a documented mechanism:
> > "-cpu Nehalem-1.0" and "-cpu Nehalem-1.1" would have the same meaning
> > every time, with any machine-type, but "-cpu Nehalem" would be an alias,
> > whose meaning depends on the machine-type.
> > 
> > Otherwise we would be stuck with a broken "Nehalem" model forever, and
> > we don't want that.
> 
> Not quite what I meant: In light of QOM we should be able to instantiate
> a CPU based on its name and optional parameters IMO. No dependency on
> the machine, please. An alias sure, but if the user explicitly says -cpu
> Nehalem then on 1.1 it should always be an alias to Nehalem-1.1 whether
> the machine is -M pc-0.15 or pc. If no -cpu was specified by the user,
> then choosing a default of Nehalem-1.0 for pc-1.0 is fine. Just trying
> to keep separate things separate here.
> 
Those things are not separate. If user will get Nehalem-1.1 with -M
pc-0.15 on qemu-1.1 it will get broken VM. If user uses -M pc-0.15
it should get exactly same machine it gets by running qemu-0.15. Guest
should not be able to tell the difference. This is the reason -M exists,
anything else is a bug.

--
			Gleb.