Re: Performance monitoring units and KVM

Avi Kivity
2008-01-30T17:41:51Z

Markus Armbruster wrote:
>
> System-wide profiling of the *virtual* machine is related to profiling
> just a process. That's hard. I guess building on Perfmon2 would make
> sense there, but as long as it's out of tree... Can we wait for it?
> If not, what then?
>
>

Give the guest access to the real PMU. Save them on every exit
(switching profiling off), and restore them on every entry (switching
profiling on). The only problem with this is that it is very cpu model
dependent, losing the hardware independence that virtual machines have.
If you are satisfied with the architectural performance counters, then
we even have hardware independence.

>
> The same ideas should work for KVM. The whole hypervisor headache
> just evaporates, of course. What remains is the host kernel routing
> samples to active guests (over virtio, I guess), and guests kernels
> receiving samples from there instead of the hardware PMU. In other
> words, the sample channel from the host becomes our virtual PMU for
> the guest. Which needs a driver for it. It's a weird PMU, because
> you can't program its performance counters. That's left to the host.
>

Is there really a requirement to profile several userspace programs, on
several guests, simultaneously? If not, passing through the PMU will
work best, with the additional advantage that guests will not need
modification (so you can run Windows with VTune, for example).

If this three-tier profiling is actually needed, perhaps we can do all
recording on the host, but have an interface to let the guest translate
rip samples to something more meaningful. This might work in this way:

- oprofile on the host receives the pmu nmi
- oprofile calls a hook (placed there by kvm) when it sees that the task
is actually a virtual machine, instead of the usual translation process
- kvm injects an interrupt into the guest
- the guest converts the pmu rip value into a meaningful string and
writes it into memory
- (later) kvm picks this up and passes it back to oprofile

The advantage here is that besides a fairly simple driver that needs to
be loaded into the guest (and can be loaded automatically), everything
is controlled from the host. All the information is available on the
host, so that sorting by counter occurences, for example, works.

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