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?
--
Andrea Bolognani / Red Hat / Virtualization