KVM: Use acquire/release semantics when accessing dirty ring GFN state

The current implementation of the dirty ring has an implicit requirement
that stores to the dirty ring from userspace must be:

- be ordered with one another

- visible from another CPU executing a ring reset

While these implicit requirements work well for x86 (and any other
TSO-like architecture), they do not work for more relaxed architectures
such as arm64 where stores to different addresses can be freely
reordered, and loads from these addresses not observing writes from
another CPU unless the required barriers (or acquire/release semantics)
are used.

In order to start fixing this, upgrade the ring reset accesses:

- the kvm_dirty_gfn_harvested() helper now uses acquire semantics
  so it is ordered after all previous writes, including that from
  userspace

- the kvm_dirty_gfn_set_invalid() helper now uses release semantics
  so that the next_slot and next_offset reads don't drift past
  the entry invalidation

This is only a partial fix as the userspace side also need upgrading.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20220926145120.27974-2-maz@kernel.org

virt/kvm/dirty_ring.c

@@ -74,7 +74,7 @@ int kvm_dirty_ring_alloc(struct kvm_dirty_ring *ring, int index, u32 size)
 
 static inline void kvm_dirty_gfn_set_invalid(struct kvm_dirty_gfn *gfn)
 {
-	gfn->flags = 0;
+	smp_store_release(&gfn->flags, 0);
 }
 
 static inline void kvm_dirty_gfn_set_dirtied(struct kvm_dirty_gfn *gfn)
@@ -84,7 +84,7 @@ static inline void kvm_dirty_gfn_set_dirtied(struct kvm_dirty_gfn *gfn)
 
 static inline bool kvm_dirty_gfn_harvested(struct kvm_dirty_gfn *gfn)
 {
-	return gfn->flags & KVM_DIRTY_GFN_F_RESET;
+	return smp_load_acquire(&gfn->flags) & KVM_DIRTY_GFN_F_RESET;
 }
 
 int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)