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

[David Ahern <dsahern@gmail.com>]

On 11/01/2011 10:18 AM, Gleb Natapov wrote:
> On Tue, Nov 01, 2011 at 09:49:27AM -0600, David Ahern wrote:
>> On 10/30/2011 10:53 AM, Gleb Natapov wrote:
>>> From: Avi Kivity <avi@redhat.com>
>>>
>>> Provide a CPUID leaf that describes the emulated PMU.
>>>
>>> Signed-off-by: Avi Kivity <avi@redhat.com>
>>> Signed-off-by: Gleb Natapov <gleb@redhat.com>
>>> ---
>>>  arch/x86/kvm/x86.c |   23 ++++++++++++++++++++++-
>>>  1 files changed, 22 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 5ea4cb8..56153a9 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -2543,6 +2543,28 @@ static void do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>>>  	}
>>>  	case 9:
>>>  		break;
>>> +	case 0xa: { /* Architectural Performance Monitoring */
>>> +		struct x86_pmu_capability cap;
>>> +
>>> +		perf_get_x86_pmu_capability(&cap);
>>> +
>>> +		/*
>>> +		 * Only support guest architectural pmu on a host
>>> +		 * with architectural pmu.
>>> +		 */
>>> +		if (!cap.version)
>>> +			memset(&cap, 0, sizeof(cap));
>>> +
>>> +		entry->eax = min(cap.version, 2)
>>> +			| (cap.num_counters_gp << 8)
>>> +			| (cap.bit_width_gp << 16)
>>> +			| (cap.events_mask_len << 24);
>>> +		entry->ebx = cap.events_mask;
>>> +		entry->ecx = 0;
>>> +		entry->edx = cap.num_counters_fixed
>>> +			| (cap.bit_width_fixed << 5);
>>> +		break;
>>> +	}
>>
>> 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.

David