5.15-stable review patch. If anyone has any objections, please let me know.

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From: Li RongQing lirongqing@baidu.com

[ Upstream commit 960550503965094b0babd7e8c83ec66c8a763b0b ]

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@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 Signed-off-by: Sasha Levin sashal@kernel.org --- 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 eba6485a59a39..4286b0f247ea3 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -959,16 +959,6 @@ ASM_RET */ 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. @@ -988,6 +978,16 @@ 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();