Re: [PATCH v3 6/9] kvm: arm/arm64: Add host pmu to support VM introspection

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Jan 18, 2017 at 04:17:18PM +0000, Punit Agrawal wrote:
> Mark Rutland <mark.rutland@arm.com> writes:
> 
> > On Wed, Jan 18, 2017 at 02:51:31PM +0000, Punit Agrawal wrote:
> >> I should've clarified in my reply that I wasn't looking to support the
> >> third instance from Mark's examples above - "monitor all vCPUs on a
> >> pCPU". I think it'll be quite expensive to figure out which threads from
> >> a given pool are vCPUs.
> >
> > I'm not sure I follow why you would need to do that?
> >
> > In that case, we'd open a CPU-bound perf event for the pCPU, which would
> > get installed in the CPU context immediately. It would be present for
> > all tasks.
> >
> > Given it's present for all tasks, we don't need to figure out which
> > happen to have vCPUs. The !vCPU tasks simply shouldn't trigger events.
> >
> > Am I missing something?
> 
> When enabling a CPU-bound event for pCPU, we'd have to enable trapping
> of TLB operations for the vCPUs running on pCPU. Have a look at Patch
> 7/9.
> 
> Also, we'd have to enable/disable trapping when tasks are migrated
> between pCPUs.

Ah, so we can't configure the trap and leave it active, since it'll
affect the host.

We could have a per-cpu flag, and a hook into vcpu_run, but that's also
gnarly.

I'll have a think.

> So far I've assumed that a VM pid is immutable. If that doesn't hold
> then we need to think of another mechanism to refer to a VM from
> userspace.

Even if we can't migrate the VM between processes (i.e. it's immutable),
it's still not unique within a process, so I'm fairly sure we need
another mechanism (even if we get away with the common case today).

Thanks,
Mark.