Re: [PATCH] KVM: VMX: Micro-optimize vmexit time when not exposing PMU

["Xu, Like" <like.xu@intel.com>]

Hi Wanpeng,

On 2020/3/12 19:05, Wanpeng Li wrote:
> On Thu, 12 Mar 2020 at 18:36, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>> Wanpeng Li <kernellwp@gmail.com> writes:
>>
>>> From: Wanpeng Li <wanpengli@tencent.com>
>>>
>>> PMU is not exposed to guest by most of cloud providers since the bad performance
>>> of PMU emulation and security concern. However, it calls perf_guest_switch_get_msrs()
>>> and clear_atomic_switch_msr() unconditionally even if PMU is not exposed to the
>>> guest before each vmentry.
>>>
>>> ~1.28% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
>>> SKX server.
>>>
>>> Before patch:
>>> vmcall 1559
>>>
>>> After patch:
>>> vmcall 1539
>>>
>>> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
>>> ---
>>>   arch/x86/kvm/vmx/vmx.c | 3 +++
>>>   1 file changed, 3 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>> index 40b1e61..fd526c8 100644
>>> --- a/arch/x86/kvm/vmx/vmx.c
>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>> @@ -6441,6 +6441,9 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
>>>        int i, nr_msrs;
>>>        struct perf_guest_switch_msr *msrs;
>>>
>>> +     if (!vcpu_to_pmu(&vmx->vcpu)->version)
>>> +             return;
>>> +
>>>        msrs = perf_guest_get_msrs(&nr_msrs);
>>>
>>>        if (!msrs)
>> Personally, I'd prefer this to be expressed as
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 40b1e6138cd5..ace92076c90f 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -6567,7 +6567,9 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>
>>          pt_guest_enter(vmx);
>>
>> -       atomic_switch_perf_msrs(vmx);
>> +       if (vcpu_to_pmu(&vmx->vcpu)->version)
We may use 'vmx->vcpu.arch.pmu.version'.

I would vote in favor of adding the "unlikely (vmx->vcpu.arch.pmu.version)"
check to the atomic_switch_perf_msrs(), which follows pt_guest_enter(vmx).

>> +               atomic_switch_perf_msrs(vmx);
>> +
> I just hope the beautiful codes before, I testing this version before
> sending out the patch, ~30 cycles can be saved which means that ~2%
> vmexit time, will update in next version. Let's wait Paolo for other
> opinions below.

You may factor the cost of the "pmu-> version check' itself (~10 cycles)
into your overall 'micro-optimize' revenue.

Thanks,
Like Xu
>
>      Wanpeng
>
>> Also, (not knowing much about PMU), is
>> "vcpu_to_pmu(&vmx->vcpu)->version" check correct?
>>
>> E.g. in intel_is_valid_msr() correct for Intel PMU or is it stated
>> somewhere that it is generic rule?
>>
>> Also, speaking about cloud providers and the 'micro' nature of this
>> optimization, would it rather make sense to introduce a static branch
>> (the policy to disable vPMU is likely to be host wide, right)?
>>
>> --
>> Vitaly
>>