Re: Performance monitoring units and KVM

Balaji Rao
2008-01-30T18:55:44Z

On Wednesday 30 January 2008 11:44:39 pm Avi Kivity wrote:
> Balaji Rao wrote:
> > But don't the architectural performance counters vary between Intel and
> > AMD cpus ? AFAIK, they do. And, this would pose problems during migration
> > between Intel and AMD hosts.
>
> The also vary between Intel hosts of different models, and likely
> different AMD hosts as well. The PMU is not architectural (or, in other
> words, model specific). So migration and PMU pass-through are mutually
> exclusive unless you have a homogeneous server farm.
>
Right. I had confused myself in understanding that Architectural Performance
monitoring is consistent across all processors right from P6. But infact it was
introduced starting with core solo and core duo.
> > I am not sure how important is it to support migration between Intel and
> > AMD hosts. If it were not that important, then IMO we could go ahead with
> > exposing the real PMU. Maybe we could warn users against running
> > profilers in the guest if they intend it to to be Intel<->AMD migrateable
> > ?
>
> We can give the user the option to expose only the architectural PMU
> (which is quite limited) and have cross-model migration, or to expose
> the full PMU and lose hardware independence.

Yes. This looks like the right thing to do..

regards,
balaji rao