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.