Re: CPUID emulation

[Stephane Eranian <eranian-sDzT885Ts8HQT0dZR+AlfA@public.gmane.org>]

Avi,

On Thu, May 31, 2007 at 11:39:04AM +0300, Avi Kivity wrote:
> Stephane Eranian wrote:
> >Hello,
> >
> >Looking at kvm-26, it seems that the CPUID values as seen by the guest OS
> >are still hardcoded for i386/x86-64 at least.
> >
> >For performance counter virtualization, the guest needs to see the *actual*
> >family/model information in order to correctly program the counters.
> >
> >It would be fairly simple to grab that information from /proc/cpuinfo
> >at init time in qemu.
> >
> >However, I am wondering if there would be side effects of using the
> >actual CPUID which could cause troubles to KVM or the guest.
> >
> >  
> 
> The main issue is migration (and save/restore).  If you migrate to a 
> host with fewer capabilities, applications may fail when they use 
> unavailable instructions that they successfully detected earlier.
> 
In the case of heterogeneous migration, clearly performance counters
will not work well, especially for unmodified guests. But I  can also
see problems when migrating from Intel Core to older P4 for instance.

> I think the best solution is to default to the host's capabilities, but 
> allow command line switches to override them.  This way a management 
> application in a server farm can set a site-wide least common 
> denominator, while a home user can enjoy the latest and greatest 
> instructions on their machine.
> 
I agree.

> Upstream qemu already has a -cpu (or similar) switch for non-x86; we can 
> probably use that.
> 

Probably, but in my particular case, you'd have to be able to specify
vendor/family/model.

> (there's another possible issue - some future features may require 
> support from the hypervisor - that may conflict with defaulting to to 
> the host feature set.  maybe kvm should mask out any unknown features)
> 

The question is how do you identify unknown features which do require
KVM support?

-- 
-Stephane

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