KVM: arm64: vgic: Explicitly implement vgic_dist::ready ordering

In preparation to remove synchronize_srcu() from MMIO registration,
remove the distributor's dependency on this implicit barrier by
direct acquire-release synchronization on the flag write and its
lock-free check.

Signed-off-by: Keir Fraser <keirf@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Keir Fraser

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

@@ -562,7 +562,7 @@ int kvm_vgic_map_resources(struct kvm *kvm)
 	gpa_t dist_base;
 	int ret = 0;
 
-	if (likely(dist->ready))
+	if (likely(smp_load_acquire(&dist->ready)))
 		return 0;
 
 	mutex_lock(&kvm->slots_lock);
@@ -593,14 +593,7 @@ int kvm_vgic_map_resources(struct kvm *kvm)
 		goto out_slots;
 	}
 
-	/*
-	 * kvm_io_bus_register_dev() guarantees all readers see the new MMIO
-	 * registration before returning through synchronize_srcu(), which also
-	 * implies a full memory barrier. As such, marking the distributor as
-	 * 'ready' here is guaranteed to be ordered after all vCPUs having seen
-	 * a completely configured distributor.
-	 */
-	dist->ready = true;
+	smp_store_release(&dist->ready, true);
 	goto out_slots;
 out:
 	mutex_unlock(&kvm->arch.config_lock);