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

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?