Re: [Qemu-arm] [PATCH V1 1/1] arm64: Add an option to turn on/off host-backed vPMU support

[Andrea Bolognani <abologna@redhat.com>]

On Mon, 2016-08-15 at 11:12 -0500, Wei Huang wrote:
> > Did you already try driving this with libvirt? It should work
> > out of the box.
> > 
> > If you haven't, I will do it :)
> 
> No, I haven't. All tests were done based on command line to QEMU. :-)

Fair enough :)

I did some testing on my own using libvirt to drive a QEMU
binary that included your patch, and it looks like everything
is working as expected: if I turn PMU on using the appropriate
XML element (<pmu state='on'/>) I can access the performance
counters from inside the guest; if I disable it explicitly or
don't enable it, I can't.

The host-side testing involved making sure the 'pmu' flag
was passed, or not passed, to QEMU and the value, if any,
matched the guest XML.

The guest-side testing involved running

  $ dmesg | grep -i pmu
  $ perf list | grep 'Hardware event'

and making sure some output was returned in both cases, and
making sure the 'instructions' counter was not marked as '<not
supported>' in the output of

  $ perf stat true

If the kind of testing I performed is not good enough, please
let me know and I'll do another round.

This was tested only with <cpu mode='host-passthrough'/> (the
libvirt equivalent of -cpu host), because the aarch64 CPU
driver in libvirt is not currently capable of handling other
CPU models for KVM guests.

I also verified that TCG guests didn't accept the 'pmu' flag.

-- 
Andrea Bolognani / Red Hat / Virtualization