Re: KVM PMU virtualization

Jes Sorensen
2010-02-26T13:38:07Z

On 02/26/10 14:31, Ingo Molnar wrote:
> You are missing two big things wrt. compatibility here:
>
> 1) The first upgrade overhead a one time overhead only.
>
> 2) Once a Linux guest has upgraded, it will work in the future, with _any_
> future CPU - _without_ having to upgrade the guest!
>
> Dont you see the advantage of that? You can instrument an old system on new
> hardware, without having to upgrade that guest for the new CPU support.

That would only work if you are guaranteed to be able to emulate old
hardware on new hardware. Not going to be feasible, so then we are in a
real mess.

> With the 'steal the PMU' messy approach the guest OS has to be upgraded to the
> new CPU type all the time. Ad infinitum.

The way the Perfmon architecture is specified by Intel, that is what we
are stuck with. It's not going to be possible via software emulation to
count cache misses, unless you run it in a micro architecture emulator.

Jes