[PATCH v6 04/21] vdpa: Avoid compiler to squash reads to used idx

["Eugenio Pérez" <eperezma@redhat.com>]

In the next patch we will allow busypolling of this value. The compiler
have a running path where shadow_used_idx, last_used_idx, and vring used
idx are not modified within the same thread busypolling.

This was not an issue before since we always cleared device event
notifier before checking it, and that could act as memory barrier.
However, the busypoll needs something similar to kernel READ_ONCE.

Let's add it here, sepparated from the polling.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/virtio/vhost-shadow-virtqueue.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index e2184a4481..560d07ab36 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -327,11 +327,12 @@ static void vhost_handle_guest_kick_notifier(EventNotifier *n)
 
 static bool vhost_svq_more_used(VhostShadowVirtqueue *svq)
 {
+    uint16_t *used_idx = &svq->vring.used->idx;
     if (svq->last_used_idx != svq->shadow_used_idx) {
         return true;
     }
 
-    svq->shadow_used_idx = cpu_to_le16(svq->vring.used->idx);
+    svq->shadow_used_idx = cpu_to_le16(*(volatile uint16_t *)used_idx);
 
     return svq->last_used_idx != svq->shadow_used_idx;
 }
-- 
2.31.1