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

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.