Re: KVM PMU virtualization

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2010-02-26 at 13:51 +0200, Avi Kivity wrote:

> It would be the other way round - the host would steal the pmu from the 
> guest.  Later we can try to time-slice and extrapolate, though that's 
> not going to be easy. 

Right, so perf already does the time slicing and interpolating thing, so
a soft-pmu gets that for free.

Anyway, this discussion seems somewhat in a stale-mate position.

The KVM folks basically demand a full PMU MSR shadow with PMI
passthrough so that their $legacy shit works without modification. 

My question with that is how $legacy muck can ever know how the current
PMU works, you can't even properly emulate a core2 pmu on a nehalem
because intel keeps messing with the event codes for every new model.

So basically for this to work means the guest can't run legacy stuff
anyway, but needs to run very up-to-date software, so we might as well
create a soft-pmu/paravirt interface now and have all up-to-date
software support that for the next generation.

Furthermore, when KVM doesn't virtualize the physical system topology,
some PMU features cannot even be sanely used from a vcpu.

So while currently a root user can already tie up all of the pmu using
perf, simply using that to hand the full pmu off to the guest still
leaves lots of issues.