Re: [PATCH MANUALSEL 5.10] KVM: x86: Yield to IPI target vCPU only if it is busy

[Paolo Bonzini]

On 3/9/22 17:46, Sasha Levin wrote:
> From: Li RongQing <lirongqing@xxxxxxxxx>
>
> [ 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@xxxxxxxxx>
> Message-Id: <1644380201-29423-1-git-send-email-lirongqing@xxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>   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 7462b79c39de..8fe6eb5bed3f 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -590,7 +590,7 @@ 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