vhost: cross-endian support for legacy devices

This patch brings cross-endian support to vhost when used to implement
legacy virtio devices. Since it is a relatively rare situation, the
feature availability is controlled by a kernel config option (not set
by default).

The vq->is_le boolean field is added to cache the endianness to be
used for ring accesses. It defaults to native endian, as expected
by legacy virtio devices. When the ring gets active, we force little
endian if the device is modern. When the ring is deactivated, we
revert to the native endian default.

If cross-endian was compiled in, a vq->user_be boolean field is added
so that userspace may request a specific endianness. This field is
used to override the default when activating the ring of a legacy
device. It has no effect on modern devices.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index a4fa33a79bf2..ce6f6da4b09f 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -106,6 +106,14 @@ struct vhost_virtqueue {
 	/* Log write descriptors */
 	void __user *log_base;
 	struct vhost_log *log;
+
+	/* Ring endianness. Defaults to legacy native endianness.
+	 * Set to true when starting a modern virtio device. */
+	bool is_le;
+#ifdef CONFIG_VHOST_CROSS_ENDIAN_LEGACY
+	/* Ring endianness requested by userspace for cross-endian support. */
+	bool user_be;
+#endif
 };
 
 struct vhost_dev {
@@ -175,8 +183,7 @@ static inline bool vhost_has_feature(struct vhost_virtqueue *vq, int bit)
 
 static inline bool vhost_is_little_endian(struct vhost_virtqueue *vq)
 {
-	return vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
-		virtio_legacy_is_little_endian();
+	return vq->is_le;
 }
 
 /* Memory accessors */