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

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Message-ID: <8fd7a83a-6fde-652f-0a2e-ec7b90c13616@redhat.com>
Date:   Mon, 17 Feb 2020 12:23:16 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Cc:     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 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)".

Thanks,

Paolo