vring: Use the DMA API on Xen

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index e46d08107a50..5c802d47892c 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -25,6 +25,7 @@
 #include <linux/hrtimer.h>
 #include <linux/kmemleak.h>
 #include <linux/dma-mapping.h>
+#include <xen/xen.h>
 
 #ifdef DEBUG
 /* For development, we want to crash whenever the ring is screwed. */
@@ -136,6 +137,17 @@ struct vring_virtqueue {
 
 static bool vring_use_dma_api(struct virtio_device *vdev)
 {
+        /*
+         * In theory, it's possible to have a buggy QEMU-supposed
+         * emulated Q35 IOMMU and Xen enabled at the same time.  On
+         * such a configuration, virtio has never worked and will
+         * not work without an even larger kludge.  Instead, enable
+         * the DMA API if we're a Xen guest, which at least allows
+         * all of the sensible Xen configurations to work correctly.
+         */
+        if (xen_domain())
+                return true;
+
         return false;
 }