On 7/22/2025 7:00 PM, lirongqing wrote:
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();
For non-overcommit VM, we may add `-overcommit cpu-pm=on` options to qemu-kvm and let guest to handle idle by itself and reduce the latency. Current kernel will fallback to virt_spin_lock, even kvm-hint-dedicated is provided. With this patch, it can fix this problem and use mcs queue spinlock for better performance.
Tested-by: Wangyang Guo <wangyang.guo@xxxxxxxxx>
