Re: [PATCH v1 0/5] KVM in-guest performance monitoring

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

On Wed, May 11, 2011 at 11:55:28AM -0400, Avi Kivity wrote:
> This not-for-merging patchset exposes an emulated version 1 architectural
> performance monitoring unit to KVM guests.  The PMU is emulated using
> perf_events, so the host kernel can multiplex host-wide, host-user, and the
> guest on available resources.
> 
> Caveats:
> - counters that have PMI (interrupt) enabled stop counting after the
>   interrupt is signalled.  This is because we need one-shot samples
>   that keep counting, which perf doesn't support yet
> - some combinations of INV and CMASK are not supported
> - counters keep on counting in the host as well as the guest
> - the RDPMC instruction and CR4.PCE bit are not yet emulated
> - there is likely a bug in the implementation; running 'perf top' in
>   a guest that spends 80% of its time in userspace shows perf itself
>   as consuming almost all cpu
> 
> perf maintainers: please consider the first three patches for merging (the
> first two make sense even without the rest).  If you're familiar with the Intel
> PMU, please review patch 5 as well - it effectively undoes all your work
> of abstracting the PMU into perf_events by unabstracting perf_events into what
> is hoped is a very similar PMU.

Gaah, I was just about to submit a talk about PMU virtualization for KVM
Forum :)

Anyway, I thought about a paravirt-approach instead of implementing a
real PMU... But there are certainly good reasons for both.

	Joerg