linux-kernel - [PATCH] x86/kvm: Reorder PV spinlock checks for dedicated CPU case

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Message-ID: <20250718094936.5283-1-lirongqing@baidu.com>
Date: Fri, 18 Jul 2025 17:49:36 +0800
From: lirongqing <lirongqing@...du.com>
To: <pbonzini@...hat.com>, <vkuznets@...hat.com>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, <hpa@...or.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
CC: Li RongQing <lirongqing@...du.com>
Subject: [PATCH] x86/kvm: Reorder PV spinlock checks for dedicated CPU case

From: Li RongQing <lirongqing@...du.com>

When a vCPU has a dedicated physical CPU, typically, the hypervisor
disables the HLT exit too, rendering the KVM_FEATURE_PV_UNHALT feature
unavailable, and virt_spin_lock_key is expected to be disabled in
this configuration, but:

The problematic execution flow caused the enabled virt_spin_lock_key:
- First check PV_UNHALT
- Then check dedicated CPUs

So change the order:
- First check dedicated CPUs
- Then check PV_UNHALT

This ensures virt_spin_lock_key is disable when dedicated physical
CPUs are available and HLT exit is disabled, and this will gives a
pretty performance boost at high contention level

Signed-off-by: Li RongQing <lirongqing@...du.com>
---
 arch/x86/kernel/kvm.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 921c1c7..9cda79f 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -1073,16 +1073,6 @@ static void kvm_wait(u8 *ptr, u8 val)
 void __init kvm_spinlock_init(void)
 {
 	/*
-	 * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
-	 * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
-	 * preferred over native qspinlock when vCPU is preempted.
-	 */
-	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
-		pr_info("PV spinlocks disabled, no host support\n");
-		return;
-	}
-
-	/*
 	 * Disable PV spinlocks and use native qspinlock when dedicated pCPUs
 	 * are available.
 	 */
@@ -1101,6 +1091,16 @@ void __init kvm_spinlock_init(void)
 		goto out;
 	}
 
+	/*
+	 * In case host doesn't support KVM_FEATURE_PV_UNHALT there is still an
+	 * advantage of keeping virt_spin_lock_key enabled: virt_spin_lock() is
+	 * preferred over native qspinlock when vCPU is preempted.
+	 */
+	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) {
+		pr_info("PV spinlocks disabled, no host support\n");
+		return;
+	}
+
 	pr_info("PV spinlocks enabled\n");
 
 	__pv_init_lock_hash();
-- 
2.9.4