[PATCH v4 2/3] KVM: X86: Choose qspinlock when dedicated vCPUs available

[Wanpeng Li]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

Waiman Long mentioned that:

 Generally speaking, unfair lock performs well for VMs with a small
 number of vCPUs. Native qspinlock may perform better than pvqspinlock
 if there is vCPU pinning and there is no vCPU over-commitment.

This patch uses a KVM_HINTS_DEDICATED performance hint to allow
hypervisor admin to choose the qspinlock to be used when a dedicated
vCPU is available.

PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock
PV_DEDICATED = 0, PV_UNHALT = 1: default is Hybrid PV queued/unfair lock
PV_DEDICATED = 0, PV_UNHALT = 0: default is tas

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Eduardo Habkost <ehabkost@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
 arch/x86/kernel/kvm.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 77a0723..c5566d9 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -733,6 +733,11 @@ void __init kvm_spinlock_init(void)
 	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
 		return;
 
+	if (kvm_hint_has_feature(KVM_HINTS_DEDICATED)) {
+		static_branch_disable(&virt_spin_lock_key);
+		return;
+	}
+
 	__pv_init_lock_hash();
 	pv_lock_ops.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
 	pv_lock_ops.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
-- 
2.7.4