For Intel, the guest PMU can be disabled via clearing the PMU CPUID.
For AMD, all hw implementations support the base set of four
performance counters, with current mainstream hardware indicating
the presence of two additional counters via X86_FEATURE_PERFCTR_CORE.

In the virtualized world, the AMD guest driver may detect
the presence of at least one counter MSR. Most hypervisor
vendors would introduce a module param (like lbrv for svm)
to disable PMU for all guests.

Another control proposal per-VM is to pass PMU disable information
via MSR_IA32_PERF_CAPABILITIES or one bit in CPUID Fn4000_00[FF:00].
Both of methods require some guest-side changes, so a module
parameter may not be sufficiently granular, but practical enough.
Signed-off-by: Like Xu <likexu@tencent.com>
Message-Id: <20211117080304.38989-1-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>