Re: [PATCH 7/9] KVM: Expose the architectural performance monitoring CPUID leaf

[David Ahern <dsahern@gmail.com>]

On 11/01/2011 10:40 AM, Gleb Natapov wrote:
>>>> If PERF_EVENTS is disabled in the host kernel will KVM return the right
>>>> thing for the guest that might have PERF_EVENTS enabled?
>>>>
>>> With correct implementation of perf_get_x86_pmu_capability() it should.
>>> PERF_EVENTS disable version should set cap.version to zero, so guest will
>>> find that vcpu does not provide architectural PMU.
>>
>> Ok.
>>
>> I was also thinking about kvm-kmod where newer KVM source is compiled
>> against older kernels - which at one point had PERF_EVENTS as a
>> configurable parameter. In this case it is possible to have perf events
>> disabled host side yet enabled guest side.
>>
> KVM PMU implementation uses host perf events to talk to HW. Without host
> perf infrastructure KVM will not be able to provide PMU to a guest. Or
> are you saying that we should support compiling KVM with !PERF_EVENTS? I'd
> rather just make KVM select PERF_EVENTS.

Strictly speaking KVM does not require PERF_EVENTS; it only requires it
if a PMU is to be offered to the guest -- which is a new KVM feature.

Not requiring PERF_EVENTS makes kvm-kmod a bit simpler, a use case where
the kvm source deals with a pre-configured kernel.

David