Re: [PATCH v2 04/10] target/i386/kvm: set KVM_PMU_CAP_DISABLE if "-pmu" is configured

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 3/5/2025 9:22 AM, Sean Christopherson wrote:
> 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.

Aha, that fix was just found by code inspection. It was not supposed to 
be related with this.

> 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");
> 
>  From the KVM side, because there's no CPUID enumeration, there's no way for KVM
> to know that userspace wants to completely disable PMU virtualization from CPUID
> alone.  Whereas with Intel CPUs, KVM infers that the PMU should be disabled by
> lack of a non-zero PMU version, e.g. if CPUID.0xA is omitted.

I see now.

Thanks to you and Dongli!

>> Anyway, since QEMU can achieve its goal with KVM_PMU_CAP_DISABLE with
>> current KVM, I'm fine with it.
> 
> Yeah, this is the only way other than disabling KVM's PMU virtualization via
> module param (enable_pmu).