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

[Xu, Like]

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'.

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