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:
-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.
On Sun, Mar 11, 2012 at 09:12:58AM -0500, Anthony Liguori wrote: 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.
This sounds pretty much like what Liu Jinsong and Jan are discussing in the TSC thread on qemu-devel. (cc'ing)
I'll look for that thread. Thanks!
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.
Note that on my qom-cpu-wip branch [1] (that I hope to have cleaned up and sent out by tomorrow), all built-in CPUs become statically registered QOM types. The external definitions that get passed in via -cpudef become dynamically registered QOM types; I took care to allow overriding existing classes with the specified -cpudef fields (but untested). Setting family, level, etc. for -cpu is done on the X86CPU object instance. [2] What I don't have yet are QOM properties to set the fields from, e.g., machine code, but those should be fairly easy to add.
Sounds interesting. I will have to take a look at the code to understand how it affects what's being discussed in this thread.
Andreas
[1] http://repo.or.cz/w/qemu/afaerber.git/shortlog/refs/heads/qom-cpu-wip
[2] http://repo.or.cz/w/qemu/afaerber.git/commit/8a6ede101a2722b790489989f21cad3...
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