linux-kernel - Re: [PATCH v2 3/3] KVM: SVM: Fix disable pause loop exit/pause filtering capability on SVM

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Date:   Thu, 30 Jul 2020 13:16:09 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2 3/3] KVM: SVM: Fix disable pause loop exit/pause filtering capability on SVM

Wanpeng Li <kernellwp@...il.com> writes:

> On Wed, 29 Jul 2020 at 20:21, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>
>> Wanpeng Li <kernellwp@...il.com> writes:
>>
>> > From: Wanpeng Li <wanpengli@...cent.com>
>> >
>> > Commit 8566ac8b (KVM: SVM: Implement pause loop exit logic in SVM) drops
>> > disable pause loop exit/pause filtering capability completely, I guess it
>> > is a merge fault by Radim since disable vmexits capabilities and pause
>> > loop exit for SVM patchsets are merged at the same time. This patch
>> > reintroduces the disable pause loop exit/pause filtering capability
>> > support.
>> >
>> > We can observe 2.9% hackbench improvement for a 92 vCPUs guest on AMD
>> > Rome Server.
>> >
>> > Reported-by: Haiwei Li <lihaiwei@...cent.com>
>> > Tested-by: Haiwei Li <lihaiwei@...cent.com>
>> > Fixes: 8566ac8b (KVM: SVM: Implement pause loop exit logic in SVM)
>> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
>> > ---
>> >  arch/x86/kvm/svm/svm.c | 9 ++++++---
>> >  1 file changed, 6 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> > index c0da4dd..c20f127 100644
>> > --- a/arch/x86/kvm/svm/svm.c
>> > +++ b/arch/x86/kvm/svm/svm.c
>> > @@ -1090,7 +1090,7 @@ static void init_vmcb(struct vcpu_svm *svm)
>> >       svm->nested.vmcb = 0;
>> >       svm->vcpu.arch.hflags = 0;
>> >
>> > -     if (pause_filter_count) {
>> > +     if (pause_filter_count && !kvm_pause_in_guest(svm->vcpu.kvm)) {
>> >               control->pause_filter_count = pause_filter_count;
>> >               if (pause_filter_thresh)
>> >                       control->pause_filter_thresh = pause_filter_thresh;
>> > @@ -2693,7 +2693,7 @@ static int pause_interception(struct vcpu_svm *svm)
>> >       struct kvm_vcpu *vcpu = &svm->vcpu;
>> >       bool in_kernel = (svm_get_cpl(vcpu) == 0);
>> >
>> > -     if (pause_filter_thresh)
>> > +     if (!kvm_pause_in_guest(vcpu->kvm))
>> >               grow_ple_window(vcpu);
>> >
>> >       kvm_vcpu_on_spin(vcpu, in_kernel);
>> > @@ -3780,7 +3780,7 @@ static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
>> >
>> >  static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu)
>> >  {
>> > -     if (pause_filter_thresh)
>> > +     if (!kvm_pause_in_guest(vcpu->kvm))
>> >               shrink_ple_window(vcpu);
>> >  }
>> >
>> > @@ -3958,6 +3958,9 @@ static void svm_vm_destroy(struct kvm *kvm)
>> >
>> >  static int svm_vm_init(struct kvm *kvm)
>> >  {
>> > +     if (!pause_filter_thresh)
>> > +             kvm->arch.pause_in_guest = true;
>>
>> Would it make sense to do
>>
>>         if (!pause_filter_count || !pause_filter_thresh)
>>                 kvm->arch.pause_in_guest = true;
>>
>> here and simplify the condition in init_vmcb()?
>
> kvm->arch.pause_in_guest can also be true when userspace sets the
> KVM_CAP_X86_DISABLE_EXITS capability, so we can't simplify the
> condition in init_vmcb().
>

I meant we simplify it to

if (!kvm_pause_in_guest(svm->vcpu.kvm))

as "!pause_filter_count" gets included.

-- 
Vitaly