x86/kvm: Don't use pvqspinlock code if only 1 vCPU

Waiman-Long · bonzini · commit 3553ae5690a8 · 2018-08-06T17:59:04.000+02:00
On a VM with only 1 vCPU, the locking fast path will always be
successful. In this case, there is no need to use the the PV qspinlock
code which has higher overhead on the unlock side than the native
qspinlock code.

Signed-off-by: Waiman Long &lt;longman@redhat.com&gt;
Signed-off-by: Paolo Bonzini &lt;pbonzini@redhat.com&gt;
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
@@ -748,6 +748,10 @@ void __init kvm_spinlock_init(void)
 	if (kvm_para_has_hint(KVM_HINTS_REALTIME))
 		return;
 
+	/* Don't use the pvqspinlock code if there is only 1 vCPU. */
+	if (num_possible_cpus() == 1)
+		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);
