KVM: dirty ring: add missing memory barrier

The KVM_DIRTY_GFN_F_DIRTY flag ensures that the entry is valid.  If
the read of the fields are not ordered after the read of the flag,
QEMU might see stale values.

Cc: Gavin Shan <gshan@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
index 8d81ab7..136c8ea 100644 (file)
--- a/accel/kvm/kvm-all.c
+++ b/accel/kvm/kvm-all.c
@@ -719,7 +719,11 @@ static void kvm_dirty_ring_mark_page(KVMState *s, uint32_t as_id,
 
 static bool dirty_gfn_is_dirtied(struct kvm_dirty_gfn *gfn)
 {
-    return gfn->flags == KVM_DIRTY_GFN_F_DIRTY;
+    /*
+     * Read the flags before the value.  Pairs with barrier in
+     * KVM's kvm_dirty_ring_push() function.
+     */
+    return qatomic_load_acquire(&gfn->flags) == KVM_DIRTY_GFN_F_DIRTY;
 }
 
 static void dirty_gfn_set_collected(struct kvm_dirty_gfn *gfn)