vhost: simplify vhost_needs_vring_endian()

After the call to virtio_vdev_has_feature(), we only care for legacy
devices, so we don't need the extra check in virtio_is_big_endian().

Also the device_endian field is always set (VIRTIO_DEVICE_ENDIAN_UNKNOWN
may only happen on a virtio_load() path that cannot lead here), so we
don't need the assert() either.

This open codes the device_endian checking in vhost_needs_vring_endian().
It also adds a comment to explain the logic, as recent reviews showed the
cross-endian tweaks aren't that obvious.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

hw/virtio/vhost.c

@@ -749,16 +749,21 @@ static void vhost_log_stop(MemoryListener *listener,
     /* FIXME: implement */
 }
 
+/* The vhost driver natively knows how to handle the vrings of non
+ * cross-endian legacy devices and modern devices. Only legacy devices
+ * exposed to a bi-endian guest may require the vhost driver to use a
+ * specific endianness.
+ */
 static inline bool vhost_needs_vring_endian(VirtIODevice *vdev)
 {
     if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
         return false;
     }
 #ifdef TARGET_IS_BIENDIAN
 #ifdef HOST_WORDS_BIGENDIAN
-    return !virtio_is_big_endian(vdev);
+    return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_LITTLE;
 #else
-    return virtio_is_big_endian(vdev);
+    return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
 #endif
 #else
     return false;