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

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 <longman@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 5b2300b..575c9a5 100644 (file)
--- 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);