linux-kernel - Re: [PATCH v3 1/2] KVM: X86: Less kvmclock sync induced vmexits after VM boots

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Message-ID: <CANRm+CxZRCs6h_mffFQsX6N7KFJFFhSADR4vn1J-1OopU+UEAA@mail.gmail.com>
Date:   Mon, 17 Feb 2020 21:12:39 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 1/2] KVM: X86: Less kvmclock sync induced vmexits after
 VM boots

On Mon, 17 Feb 2020 at 19:23, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 17/02/20 11:36, Wanpeng Li wrote:
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index fb5d64e..d0ba2d4 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9390,8 +9390,9 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
> >      if (!kvmclock_periodic_sync)
> >          return;
> >
> > -    schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
> > -                    KVMCLOCK_SYNC_PERIOD);
> > +    if (kvm->created_vcpus == 1)
> > +        schedule_delayed_work(&kvm->arch.kvmclock_sync_work,
> > +                        KVMCLOCK_SYNC_PERIOD);
>
> This is called with kvm->lock not held, so you can have
> kvm->created_vcpus == 2 by the time you get here.  You can test instead
> "if (vcpu->vcpu_idx == 0)".

Agreed.

    Wanpeng