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Date:   Thu, 8 Apr 2021 16:56:22 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH] KVM: X86: Do not yield to self
Message-ID: <YG81to0nF/M7DEGA@google.com>
References: <1617880989-8019-1-git-send-email-wanpengli@tencent.com>
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On Thu, Apr 08, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> If the target is self we do not need to yield, we can avoid malicious 
> guest to play this.
> 
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> Rebased on https://lore.kernel.org/kvm/1617697935-4158-1-git-send-email-wanpengli@tencent.com/
> 
>  arch/x86/kvm/x86.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 43c9f9b..260650f 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8230,6 +8230,10 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
>  	if (!target)
>  		goto no_yield;
>  
> +	/* yield to self */

If you're going to bother with a comment, maybe elaborate a bit, e.g.

	/* Ignore requests to yield to self. */

> +	if (vcpu->vcpu_id == target->vcpu_id)
> +		goto no_yield;
> +
>  	if (!READ_ONCE(target->ready))
>  		goto no_yield;
>  
> -- 
> 2.7.4
>