linux-kernel - Re: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU

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Date:   Tue, 19 Oct 2021 18:59:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU

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;
> +	}