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

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.