[SRU][Q:gcp][PATCH 5/5] x86/kvm: Prefer native qspinlock for dedicated vCPUs irrespective of PV_UNHALT

[Tim Whisonant]

From: Li RongQing <lirongqing at baidu.com>

BugLink: https://bugs.launchpad.net/bugs/2133834

The commit b2798ba0b876 ("KVM: X86: Choose qspinlock when dedicated
physical CPUs are available") states that when PV_DEDICATED=1
(vCPU has dedicated pCPU), qspinlock should be preferred regardless of
PV_UNHALT.  However, the current implementation doesn't reflect this: when
PV_UNHALT=0, we still use virt_spin_lock() even with dedicated pCPUs.

This is suboptimal because:
1. Native qspinlocks should outperform virt_spin_lock() for dedicated
   vCPUs irrespective of HALT exiting
2. virt_spin_lock() should only be preferred when vCPUs may be preempted
   (non-dedicated case)

So reorder the PV spinlock checks to:
1. First handle dedicated pCPU case (disable virt_spin_lock_key)
2. Second check single CPU, and nopvspin configuration
3. Only then check PV_UNHALT support

This ensures we always use native qspinlock for dedicated vCPUs, delivering
pretty performance gains at high contention levels.

Signed-off-by: Li RongQing <lirongqing at baidu.com>
Reviewed-by: Sean Christopherson <seanjc at google.com>
Tested-by: Wangyang Guo <wangyang.guo at intel.com>
Link: https://lore.kernel.org/r/20250722110005.4988-1-lirongqing@baidu.com
Signed-off-by: Sean Christopherson <seanjc at google.com>
(cherry picked from commit 960550503965094b0babd7e8c83ec66c8a763b0b)
Signed-off-by: Tim Whisonant <tim.whisonant at canonical.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 56608d4d735f..b67d7c59dca0 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -1088,16 +1088,6 @@ 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.
@@ -1117,6 +1107,16 @@ 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();
-- 
2.43.0