On 3/23/20 2:28 PM, Andrea Bolognani wrote:
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.
I'll check it out how <panic/> is implemented. As long as we don't end up putting "Property '.pmu' not found" in the QEMU command line (which will cause an error) then I think it's ok to have it in the XML.
David, what's your opinion on the matter?