Commit 96055050 authored Jul 22, 2025 by Li RongQing Committed by Sean Christopherson Sep 11, 2025

x86/kvm: Prefer native qspinlock for dedicated vCPUs irrespective of PV_UNHALT



The commit b2798ba0 ("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@baidu.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Tested-by: Wangyang Guo <wangyang.guo@intel.com>
Link: https://lore.kernel.org/r/20250722110005.4988-1-lirongqing@baidu.com


Signed-off-by: Sean Christopherson <seanjc@google.com>

parent 657bf704

arch/x86/kernel/kvm.c

+10 −10

Original line number	Diff line number	Diff line
		@@ -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();