linux-kernel - [RFC] x86/kvm: Use native qspinlock by default when realtime hinted

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Message-Id: <20250702064218.894-1-liangyan.peng@bytedance.com>
Date: Wed,  2 Jul 2025 14:42:18 +0800
From: Liangyan <liangyan.peng@...edance.com>
To: pbonzini@...hat.com,
	vkuznets@...hat.com,
	tglx@...utronix.de,
	mingo@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	hpa@...or.com
Cc: linux-kernel@...r.kernel.org,
	x86@...nel.org,
	kvm@...r.kernel.org,
	Liangyan <liangyan.peng@...edance.com>
Subject: [RFC] x86/kvm: Use native qspinlock by default when realtime hinted

When KVM_HINTS_REALTIME is set and KVM_FEATURE_PV_UNHALT is clear,
currently guest will use virt_spin_lock.
Since KVM_HINTS_REALTIME is set, use native qspinlock should be safe
and have better performance than virt_spin_lock.

Signed-off-by: Liangyan <liangyan.peng@...edance.com>
---
 arch/x86/kernel/kvm.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 921c1c783bc1..9080544a4007 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -1072,6 +1072,15 @@ static void kvm_wait(u8 *ptr, u8 val)
  */
 void __init kvm_spinlock_init(void)
 {
+	/*
+	 * Disable PV spinlocks and use native qspinlock when dedicated pCPUs
+	 * are available.
+	 */
+	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
+		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints\n");
+		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
@@ -1082,15 +1091,6 @@ void __init kvm_spinlock_init(void)
 		return;
 	}
 
-	/*
-	 * Disable PV spinlocks and use native qspinlock when dedicated pCPUs
-	 * are available.
-	 */
-	if (kvm_para_has_hint(KVM_HINTS_REALTIME)) {
-		pr_info("PV spinlocks disabled with KVM_HINTS_REALTIME hints\n");
-		goto out;
-	}
-
 	if (num_possible_cpus() == 1) {
 		pr_info("PV spinlocks disabled, single CPU\n");
 		goto out;
-- 
2.39.3 (Apple Git-145)