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

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

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;
> +	}