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Message-ID: <24e67e43-c50c-7e0f-305a-c7f6129f8d70@redhat.com>
Date:   Tue, 19 Oct 2021 18:59:43 +0200
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Subject: Re: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU
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To:     Wanpeng Li <kernellwp@gmail.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
References: <1634631160-67276-1-git-send-email-wanpengli@tencent.com>
 <1634631160-67276-3-git-send-email-wanpengli@tencent.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <1634631160-67276-3-git-send-email-wanpengli@tencent.com>
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On 19/10/21 10:12, Wanpeng Li wrote:
> -	if (kvm_vcpu_wake_up(vcpu))
> -		return;
> +	me = get_cpu();
> +
> +	if (rcuwait_active(kvm_arch_vcpu_get_wait(vcpu)) && kvm_vcpu_wake_up(vcpu))
> +		goto out;

This is racy.  You are basically doing the same check that 
rcuwait_wake_up does, but without the memory barrier before.

Also here:

> +	if (vcpu == __this_cpu_read(kvm_running_vcpu)) {
> +		WARN_ON_ONCE(vcpu->mode == IN_GUEST_MODE);

it's better to do

	if (vcpu == ... && !WARN_ON_ONCE(vcpu->mode == IN_GUEST_MODE))
		goto out;

so that if the bug happens you do get a smp_send_reschedule() and fail 
safely.

Paolo

> +		goto out;
> +	}