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

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[ Upstream commit 960550503965094b0babd7e8c83ec66c8a763b0b ]

The commit b2798ba0b8 ("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>
This commit is contained in:
Li RongQing
2025-07-22 19:00:05 +08:00
committed by Greg Kroah-Hartman
parent 1a6ffaeddf
commit 2ddd6bf6b6
1 changed files with 10 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

20
arch/x86/kernel/kvm.c
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@@ -1066,16 +1066,6 @@ 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.
@@ -1095,6 +1085,16 @@ 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();