From: Li RongQing <lirongqing@xxxxxxxxx>
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@xxxxxxxxx>
---
diff with v1: rewrite the changelog
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
