linux-kernel - Re: [PATCH MANUALSEL 5.16] KVM: x86: Yield to IPI target vCPU only if it is busy

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Message-ID: <c7e4a000-9e76-266a-7cb3-4ccdaf54bf6b@redhat.com>
Date:   Wed, 9 Mar 2022 17:47:03 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Li RongQing <lirongqing@...du.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.16] KVM: x86: Yield to IPI target vCPU only if
 it is busy

On 3/9/22 17:46, Sasha Levin wrote:
> From: Li RongQing <lirongqing@...du.com>
> 
> [ Upstream commit 9ee83635d872812f3920209c606c6ea9e412ffcc ]
> 
> When sending a call-function IPI-many to vCPUs, yield to the
> IPI target vCPU which is marked as preempted.
> 
> but when emulating HLT, an idling vCPU will be voluntarily
> scheduled out and mark as preempted from the guest kernel
> perspective. yielding to idle vCPU is pointless and increase
> unnecessary vmexit, maybe miss the true preempted vCPU
> 
> so yield to IPI target vCPU only if vCPU is busy and preempted
> 
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> Message-Id: <1644380201-29423-1-git-send-email-lirongqing@...du.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kernel/kvm.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 59abbdad7729..2121c20e877f 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -619,7 +619,7 @@ static void kvm_smp_send_call_func_ipi(const struct cpumask *mask)
>   
>   	/* Make sure other vCPUs get a chance to run if they need to. */
>   	for_each_cpu(cpu, mask) {
> -		if (vcpu_is_preempted(cpu)) {
> +		if (!idle_cpu(cpu) && vcpu_is_preempted(cpu)) {
>   			kvm_hypercall1(KVM_HC_SCHED_YIELD, per_cpu(x86_cpu_to_apicid, cpu));
>   			break;
>   		}

NACK, just an optimization.

Paolo