Re: Performance monitoring units and KVM

[Balaji Rao <balajirrao-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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

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