vhost: fix double read of descriptor flags

Flags could be updated in a separate process leading to the
inconsistent check.

Additionally, read marked as 'volatile' to highlight the shared
nature of the variable and avoid such issues in the future.

Fixes: d3211c98c456 ("vhost: add helpers for packed virtqueues")
Cc: stable@dpdk.org
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>

diff --git a/lib/librte_vhost/vhost.h b/lib/librte_vhost/vhost.h
index 5218f1b..552b929 100644 (file)
--- a/lib/librte_vhost/vhost.h
+++ b/lib/librte_vhost/vhost.h
@@ -393,8 +393,10 @@ vq_is_packed(struct virtio_net *dev)
 static inline bool
 desc_is_avail(struct vring_packed_desc *desc, bool wrap_counter)
 {
-       return wrap_counter == !!(desc->flags & VRING_DESC_F_AVAIL) &&
-               wrap_counter != !!(desc->flags & VRING_DESC_F_USED);
+       uint16_t flags = *((volatile uint16_t *) &desc->flags);
+
+       return wrap_counter == !!(flags & VRING_DESC_F_AVAIL) &&
+               wrap_counter != !!(flags & VRING_DESC_F_USED);
 }
 
 #define VHOST_LOG_PAGE 4096