[PATCH 2/3] KVM: arm64: vgic: Explicitly implement vgic_dist::ready ordering

[Keir Fraser <keirf@google.com>]

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>
---
 arch/arm64/kvm/vgic/vgic-init.c | 11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
index 502b65049703..bc83672e461b 100644
--- a/arch/arm64/kvm/vgic/vgic-init.c
+++ b/arch/arm64/kvm/vgic/vgic-init.c
@@ -567,7 +567,7 @@ 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);
@@ -598,14 +598,7 @@ 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);
-- 
2.50.0.rc2.761.g2dc52ea45b-goog