linux-kernel - [PATCH MANUALSEL 5.4] KVM: x86: Yield to IPI target vCPU only if it is busy

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Date:   Wed,  9 Mar 2022 11:46:55 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Li RongQing <lirongqing@...du.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sasha Levin <sashal@...nel.org>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: [PATCH MANUALSEL 5.4] KVM: x86: Yield to IPI target vCPU only if it is busy

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 6ff2c7cac4c4..77e4d875a468 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -543,7 +543,7 @@ 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;
 		}
-- 
2.34.1