[PATCH v1 3/4] qemu_domain.c: do not launch ppc64 guests with 'pmu' setting

[Andrea Bolognani]

On Thu, 2020-03-19 at 18:44 -0300, Daniel Henrique Barboza wrote:
> The Perfomance Monitoring Unit (PMU) feature is not available for
> the Power architecture. The "<pmu/>" feature will always have a value
> 'on' or 'off' after saving the domain XML, and both will be rejected
> by QEMU when launching. This is the error message for
> "<pmu state='on'/>":
> 
> qemu-kvm: can't apply global host-powerpc64-cpu.pmu=on: Property '.pmu' not found
> 
> A similar error message is thrown for "<pmu state='off'/>".
> 
> This patch prevents the pseries guest from launching with any
> pmu setting with a more informative error message:
> 
> error: unsupported configuration: The 'pmu' feature is not
> supported for architecture 'ppc64' or machine type 'pseries'

I don't think this is right. While you are correct that PMU can't be
configured for pSeries guests, I think that's because of the opposite
reason: it's always on, and can't be turned off.

For comparison's sake: in an x86 guest with <pmu state='on'/> I get

  $ perf list | grep -E 'Hardware.*event'
    branch-instructions OR branches  [Hardware event]
    branch-misses                    [Hardware event]
    bus-cycles                       [Hardware event]
    cache-misses                     [Hardware event]
    cache-references                 [Hardware event]
    cpu-cycles OR cycles             [Hardware event]
    instructions                     [Hardware event]
    ref-cycles                       [Hardware event]
    L1-dcache-load-misses            [Hardware cache event]
    L1-dcache-loads                  [Hardware cache event]
    L1-dcache-stores                 [Hardware cache event]
    L1-icache-load-misses            [Hardware cache event]
    branch-load-misses               [Hardware cache event]
    branch-loads                     [Hardware cache event]
    dTLB-load-misses                 [Hardware cache event]
    dTLB-loads                       [Hardware cache event]
    dTLB-store-misses                [Hardware cache event]
    dTLB-stores                      [Hardware cache event]
    iTLB-load-misses                 [Hardware cache event]
    iTLB-loads                       [Hardware cache event]

whereas when I turn off PMU all of those are gone.

In a pSeries guest running on POWER8, without any configuration,
I get:

  $ perf list | grep -E 'Hardware.*event'
    branch-instructions OR branches                  [Hardware event]
    branch-misses                                    [Hardware event]
    cache-misses                                     [Hardware event]
    cache-references                                 [Hardware event]
    cpu-cycles OR cycles                             [Hardware event]
    instructions                                     [Hardware event]
    stalled-cycles-backend OR idle-cycles-backend    [Hardware event]
    stalled-cycles-frontend OR idle-cycles-frontend  [Hardware event]
    L1-dcache-load-misses                            [Hardware cache event]
    L1-dcache-loads                                  [Hardware cache event]
    L1-dcache-prefetches                             [Hardware cache event]
    L1-dcache-store-misses                           [Hardware cache event]
    L1-icache-load-misses                            [Hardware cache event]
    L1-icache-loads                                  [Hardware cache event]
    L1-icache-prefetches                             [Hardware cache event]
    LLC-load-misses                                  [Hardware cache event]
    LLC-loads                                        [Hardware cache event]
    LLC-prefetches                                   [Hardware cache event]
    LLC-store-misses                                 [Hardware cache event]
    LLC-stores                                       [Hardware cache event]
    branch-load-misses                               [Hardware cache event]
    branch-loads                                     [Hardware cache event]
    dTLB-load-misses                                 [Hardware cache event]
    iTLB-load-misses                                 [Hardware cache event]

So it seems to me that, if anything, the PMU feature should be
treated like the <panic/> device, that is, automatically added to
pSeries guests if it's not present already.

David, what's your opinion on the matter?

-- 
Andrea Bolognani / Red Hat / Virtualization