On Tue, Mar 04, 2025, Xiaoyao Li wrote:
> On 3/3/2025 6:00 AM, Dongli Zhang wrote:
> > Although AMD PERFCORE and PerfMonV2 are removed when "-pmu" is configured,
> > there is no way to fully disable KVM AMD PMU virtualization. Neither
> > "-cpu host,-pmu" nor "-cpu EPYC" achieves this.
>
> This looks like a KVM bug.
Heh, the patches you sent do fix _a_ KVM bug, but this is something else entirely.
In practice, the KVM bug only affects what KVM_GET_SUPPORTED_CPUID returns when
enable_pmu=false, and in that case, it's only a reporting issue, i.e. KVM will
still block usage of the PMU.
As Dongli pointed out, older AMD CPUs don't actually enumerate a PMU in CPUID,
and so the kernel assumes that not-too-old CPUs have a PMU:
/* Performance-monitoring supported from K7 and later: */
if (boot_cpu_data.x86 < 6)
return -ENODEV;
The "expected" output:
Performance Events: PMU not available due to virtualization, using software events only.
is a long-standing workaround in the kernel to deal with lack of enumeration. On
top of explicit enumeration, init_hw_perf_events() => check_hw_exists() probes
hardware to see if it actually works. If an MSR is unexpectedly unavailable, as
is the case when running as a guest, the kernel prints a message and disables PMU
usage. E.g. the above message is specific to running as a guest:
if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
pr_cont("PMU not available due to virtualization, using software events only.\n");
