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

From: Andreas Färber
Subject: Re: [Qemu-devel] [libvirt] Modern CPU models cannot be used with libvirt
Date: Mon, 12 Mar 2012 16:49:47 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.2) Gecko/20120215 Thunderbird/10.0.2

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.

This sounds pretty much like what Liu Jinsong and Jan are discussing in
the TSC thread on qemu-devel. (cc'ing)

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.

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.


[1] http://repo.or.cz/w/qemu/afaerber.git/shortlog/refs/heads/qom-cpu-wip


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