kvm tools: Fix virt_queue__set_used_elem

Increase idx only after updating the used element.
Not doing so may mark a buffer as used without having
it's head and length updated.

Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

virtio.c

@@ -1,15 +1,32 @@
 #include <linux/virtio_ring.h>
 #include <stdint.h>
 #include <sys/uio.h>
+#include <asm/system.h>
 #include "kvm/kvm.h"
 #include "kvm/virtio.h"
 
 struct vring_used_elem *virt_queue__set_used_elem(struct virt_queue *queue, uint32_t head, uint32_t len)
 {
 	struct vring_used_elem *used_elem;
-	used_elem	= &queue->vring.used->ring[queue->vring.used->idx++ % queue->vring.num];
+	used_elem	= &queue->vring.used->ring[queue->vring.used->idx % queue->vring.num];
 	used_elem->id	= head;
 	used_elem->len	= len;
+
+	/*
+	 * Use wmb to assure that used elem was updated with head and len.
+	 * We need a wmb here since we can't advance idx unless we're ready
+	 * to pass the used element to the guest.
+	 */
+	wmb();
+	queue->vring.used->idx++;
+
+	/*
+	 * Use wmb to assure used idx has been increased before we signal the guest.
+	 * Without a wmb here the guest may ignore the queue since it won't see
+	 * an updated idx.
+	 */
+	wmb();
+
 	return used_elem;
 }