KVM: X86: Choose qspinlock when dedicated physical CPUs are available

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 the KVM_HINTS_DEDICATED performance hint, which is
provided by the hypervisor admin, to choose the qspinlock algorithm
when a dedicated physical CPU 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@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 8c9d98c..f803e89 100644 (file)
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -735,6 +735,11 @@ void __init kvm_spinlock_init(void)
        if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
                return;
 
+       if (kvm_para_has_hint(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);