Re: CPUID emulation

[Troy Benjegerdes <hozer-TBByXz/9jYzYtjvyW6yDsg@public.gmane.org>]

On Thu, May 31, 2007 at 04:58:41AM -0700, Stephane Eranian wrote:
> Avi,
> 
> On Thu, May 31, 2007 at 02:52:05PM +0300, Avi Kivity wrote:
> > Stephane Eranian wrote:
> > >>
> > >>If the guest cpuid is set to a least common denominator, it should work.
> > >>
> > >>    
> > >There is no common denominator between a P4 and Intel Core 2 Duo for the
> > >performance counters. So you cannot simply use a generic member of family
> > >15 to fake the guest cpuid runnning on Intel Core 2 Duo host.
> > >
> > >  
> > 
> > So, the performance counter functionality will not be available if you 
> > have a mixed server farm with these processors.
> > 
> That's like what is going to happen.
> 
> > If applications use model version to detect performance counters, and 
> > not cpuid bits, then there is no way to prevent guests using performance 
> > counters.  Fortunately this is limited to specialized applications.
> > 
> They use cpuid. I expect more and more applications/OS will rely on
> performance counters to boost performance at runtime.

This sounds like a huge headache waiting to happen, and something that
could end up giving virtualization and KVM a bad name in the long run,
due to the inevitable bugs and performance problems that will happen
with end-users that don't know the details about what performance
counters are supported on what cpu model.

The only halfway sane solution I can think of involves having guest
support for CPU hotplug, so that a host can notify a guest that the
underlying CPU has changed.

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