Re: KVM PMU virtualization

[Joerg Roedel <joro@8bytes.org>]

On Mon, Mar 01, 2010 at 09:39:04AM +0100, Ingo Molnar wrote:
> > What do you mean by software events?
> 
> Things like:
> 
> aldebaran:~> perf stat -a sleep 1
> 
>  Performance counter stats for 'sleep 1':
> 
>    15995.719133  task-clock-msecs         #     15.981 CPUs 
>            5787  context-switches         #      0.000 M/sec
>             210  CPU-migrations           #      0.000 M/sec
>          193909  page-faults              #      0.012 M/sec
>     28704833507  cycles                   #   1794.532 M/sec  (scaled from 78.69%)
>     14387445668  instructions             #      0.501 IPC    (scaled from 90.71%)
>       736644616  branches                 #     46.053 M/sec  (scaled from 90.52%)
>       695884659  branch-misses            #     94.467 %      (scaled from 90.70%)
>       727070678  cache-references         #     45.454 M/sec  (scaled from 88.11%)
>      1305560420  cache-misses             #     81.619 M/sec  (scaled from 52.00%)
> 
>     1.000942399  seconds time elapsed
> 
> These lines:
> 
>    15995.719133  task-clock-msecs         #     15.981 CPUs 
>            5787  context-switches         #      0.000 M/sec
>             210  CPU-migrations           #      0.000 M/sec
>          193909  page-faults              #      0.012 M/sec
> 
> Are software events of the host - a subset of which could be transparently 
> exposed to the guest. Same for tracepoints, probes, etc. Those are not exposed 
> by the hardware PMU. So by doing a 'soft' PMU (or even better: a paravirt 
> channel to perf events) we gain a lot more than just raw PMU functionality.
> 
> 'performance events' are about a lot more than just the PMU, it's a coherent 
> system health / system events / structured logging framework.

Yeah I know. But these event should be available in the guest already,
no? They don't need any kind of hardware support from the pmu.
A paravirt perf channel from the guest to the host would be definitly a
win. It would be a powerful tool for kvm/linux-guest analysis (e.g.
trace host-kvm and guest-events together on the host)

	Joerg