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

[Wanpeng Li]

On Fri, 13 Mar 2020 at 11:23, Xu, Like <like.xu@xxxxxxxxx> wrote:
>
> Hi Wanpeng,
>
> On 2020/3/12 19:05, Wanpeng Li wrote:
> > On Thu, 12 Mar 2020 at 18:36, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> >> Wanpeng Li <kernellwp@xxxxxxxxx> writes:
> >>
> >>> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >>>
> >>> 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@xxxxxxxxxxx>
> >>> ---
> >>>   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'.

Thanks for confirm this. Maybe this is better:

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e61..b20423c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6567,7 +6567,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)

        pt_guest_enter(vmx);

-       atomic_switch_perf_msrs(vmx);
+       if (vcpu_to_pmu(vcpu)->version)
+               atomic_switch_perf_msrs(vmx);
        atomic_switch_umwait_control_msr(vmx);

        if (enable_preemption_timer)

>
> 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).

This is hotpath, let's save the cost of function call.

    Wanpeng

>
> >> +               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
> >>
>