vhost: logging thinko fix

vhost was dong some complex math to get
offset to log at, and got it wrong by a couple of bytes,
while in fact it's simple: get address where we write,
subtract start of buffer, add log base.

Do it this way.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 6eb15259f5ae..db21518f7e8d 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1004,10 +1004,14 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
         if (unlikely(vq->log_used)) {
                 /* Make sure data is seen before log. */
                 smp_wmb();
-                log_write(vq->log_base, vq->log_addr + sizeof *vq->used->ring *
-                          (vq->last_used_idx % vq->num),
-                          sizeof *vq->used->ring);
-                log_write(vq->log_base, vq->log_addr, sizeof *vq->used->ring);
+                /* Log used ring entry write. */
+                log_write(vq->log_base,
+                          vq->log_addr + ((void *)used - (void *)vq->used),
+                          sizeof *used);
+                /* Log used index update. */
+                log_write(vq->log_base,
+                          vq->log_addr + offsetof(struct vring_used, idx),
+                          sizeof vq->used->idx);
                 if (vq->log_ctx)
                         eventfd_signal(vq->log_ctx, 1);
         }