On 11/10/23 01:01, Dongli Zhang wrote:
On 11/9/23 3:46 PM, Denis V. Lunev wrote:
On 11/9/23 23:52, Jim Mattson wrote:
On Thu, Nov 9, 2023 at 10:18 AM Konstantin Khorenko
<khorenko@xxxxxxxxxxxxx> wrote:
Hi All,
as a followup for my patch: i have noticed that
currently Intel kernel code provides an ability to detect if PMU is totally
disabled for a VM
(pmu->version == 0 in this case), but for AMD code pmu->version is never 0,
no matter if PMU is enabled or disabled for a VM (i mean <pmu state='off'/>
in the VM config which
results in "-cpu pmu=off" qemu option).
So the question is - is it possible to enhance the code for AMD to also honor
PMU VM setting or it is
impossible by design?
The AMD architectural specification prior to AMD PMU v2 does not allow
one to describe a CPU (via CPUID or MSRs) that has fewer than 4
general purpose PMU counters. While AMD PMU v2 does allow one to
describe such a CPU, legacy software that knows nothing of AMD PMU v2
can expect four counters regardless.
Having said that, KVM does provide a per-VM capability for disabling
the virtual PMU: KVM_CAP_PMU_CAPABILITY(KVM_PMU_CAP_DISABLE). See
section 8.35 in Documentation/virt/kvm/api.rst.
But this means in particular that QEMU should immediately
use this KVM_PMU_CAP_DISABLE if this capability is supported and PMU=off. I am
not seeing this code thus I believe that we have missed this. I think that this
change worth adding. We will measure the impact :-) Den
I used to have a patch to use KVM_PMU_CAP_DISABLE in QEMU, but that did not draw
many developers' attention.
https://lore.kernel.org/qemu-devel/20230621013821.6874-2-dongli.zhang@xxxxxxxxxx/
It is time to first re-send that again.
Dongli Zhang
We have checked that setting KVM_PMU_CAP_DISABLE really helps.
Konstantin has done this and this is good. On the other hand, looking
into these patches I disagree with them. We should not introduce new
option for QEMU. If PMU is disabled, i.e. we assume that pmu=off passed
in the command line, we should set KVM_PMU_CAP_DISABLE for that virtual
machine. Den
