KVM: arm64: vgic-v3: Limit IPI-ing when accessing GICR_{C,S}ACTIVER0

When a vcpu is accessing *its own* redistributor's SGIs/PPIs, there
is no point in doing a stop-the-world operation. Instead, we can
just let the access occur as we do with GICv2.

This is a very minor optimisation for a non-nesting guest, but
a potentially major one for a nesting L1 hypervisor which is
likely to access the emulated registers pretty often (on each
vcpu switch, at the very least).

Reported-by: Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230112154840.1808595-1-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/vgic/vgic-mmio.c

@@ -473,9 +473,10 @@ int vgic_uaccess_write_cpending(struct kvm_vcpu *vcpu,
  * active state can be overwritten when the VCPU's state is synced coming back
  * from the guest.
  *
- * For shared interrupts as well as GICv3 private interrupts, we have to
- * stop all the VCPUs because interrupts can be migrated while we don't hold
- * the IRQ locks and we don't want to be chasing moving targets.
+ * For shared interrupts as well as GICv3 private interrupts accessed from the
+ * non-owning CPU, we have to stop all the VCPUs because interrupts can be
+ * migrated while we don't hold the IRQ locks and we don't want to be chasing
+ * moving targets.
  *
  * For GICv2 private interrupts we don't have to do anything because
  * userspace accesses to the VGIC state already require all VCPUs to be
@@ -484,7 +485,8 @@ int vgic_uaccess_write_cpending(struct kvm_vcpu *vcpu,
  */
 static void vgic_access_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
 {
-	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3 ||
+	if ((vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3 &&
+	     vcpu != kvm_get_running_vcpu()) ||
 	    intid >= VGIC_NR_PRIVATE_IRQS)
 		kvm_arm_halt_guest(vcpu->kvm);
 }
@@ -492,7 +494,8 @@ static void vgic_access_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
 /* See vgic_access_active_prepare */
 static void vgic_access_active_finish(struct kvm_vcpu *vcpu, u32 intid)
 {
-	if (vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3 ||
+	if ((vcpu->kvm->arch.vgic.vgic_model == KVM_DEV_TYPE_ARM_VGIC_V3 &&
+	     vcpu != kvm_get_running_vcpu()) ||
 	    intid >= VGIC_NR_PRIVATE_IRQS)
 		kvm_arm_resume_guest(vcpu->kvm);
 }