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.
Signed-off-by: Sasha Levin <levinsasha928@xxxxxxxxx>
---
tools/kvm/virtio.c | 19 ++++++++++++++++++-
1 files changed, 18 insertions(+), 1 deletions(-)
diff --git a/tools/kvm/virtio.c b/tools/kvm/virtio.c
index 6249521..266a1b6 100644
--- a/tools/kvm/virtio.c
+++ b/tools/kvm/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;
}
--
1.7.5.rc3
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