Re: KVM PMU virtualization

[Ingo Molnar <mingo@elte.hu>]

* Joerg Roedel <joro@8bytes.org> wrote:

> >  - It's more secure: the host can have a finegrained policy about what kinds of
> >    events it exposes to the guest. It might chose to only expose software 
> >    events for example.
> 
> 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.

	Ingo