[Qemu-devel] [RFC PATCH 1/2] vhost: add VHOST_VRING_F_BYTESWAP flag

[Cédric Le Goater]

When the guest and the host have a different endian order, the data 
being accessed in the vring queues needs to be byteswapped.

This patch adds a VHOST_VRING_F_BYTESWAP flag to inform the vhost 
kernel backend to byteswap vring data.

Signed-off-by: Cédric Le Goater <address@hidden>
---
 hw/virtio/vhost.c           |   25 ++++++++++++++++++++++++-
 include/hw/virtio/vhost.h   |    1 +
 linux-headers/linux/vhost.h |    3 +++
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 5d7c40ac04d0..75308d127d90 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -539,7 +539,13 @@ static int vhost_virtqueue_set_addr(struct vhost_dev *dev,
         .log_guest_addr = vq->used_phys,
         .flags = enable_log ? (1 << VHOST_VRING_F_LOG) : 0,
     };
-    int r = dev->vhost_ops->vhost_call(dev, VHOST_SET_VRING_ADDR, &addr);
+    int r;
+
+    if (vq->byteswap) {
+       addr.flags |= (1 << VHOST_VRING_F_BYTESWAP);
+    }
+
+    r = dev->vhost_ops->vhost_call(dev, VHOST_SET_VRING_ADDR, &addr);
     if (r < 0) {
         return -errno;
     }
@@ -647,6 +653,21 @@ static void vhost_log_stop(MemoryListener *listener,
     /* FIXME: implement */
 }
 
+
+static bool vhost_virtqueue_needs_byteswap(VirtIODevice *vdev)
+{
+#ifdef TARGET_IS_BIENDIAN
+#ifdef HOST_WORDS_BIGENDIAN
+    return !virtio_is_big_endian(vdev);
+#else
+    return virtio_is_big_endian(vdev);
+#endif
+
+#else
+    return false;
+#endif
+}
+
 static int vhost_virtqueue_start(struct vhost_dev *dev,
                                 struct VirtIODevice *vdev,
                                 struct vhost_virtqueue *vq,
@@ -707,6 +728,8 @@ static int vhost_virtqueue_start(struct vhost_dev *dev,
         goto fail_alloc_ring;
     }
 
+    vq->byteswap = vhost_virtqueue_needs_byteswap(vdev);
+
     r = vhost_virtqueue_set_addr(dev, vq, vhost_vq_index, dev->log_enabled);
     if (r < 0) {
         r = -errno;
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index d5593d162079..793f89d71ffb 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -20,6 +20,7 @@ struct vhost_virtqueue {
     unsigned long long ring_phys;
     unsigned ring_size;
     EventNotifier masked_notifier;
+    bool byteswap;
 };
 
 typedef unsigned long vhost_log_chunk_t;
diff --git a/linux-headers/linux/vhost.h b/linux-headers/linux/vhost.h
index c656f61cfc75..0bbd3f1209f3 100644
--- a/linux-headers/linux/vhost.h
+++ b/linux-headers/linux/vhost.h
@@ -34,6 +34,9 @@ struct vhost_vring_addr {
        /* Flag values: */
        /* Whether log address is valid. If set enables logging. */
 #define VHOST_VRING_F_LOG 0
+       /* Whether vring memory accesses should be byte-swapped.
+        * required when the guest has a different endianess */
+#define VHOST_VRING_F_BYTESWAP 1
 
        /* Start of array of descriptors (virtually contiguous) */
        __u64 desc_user_addr;
-- 
1.7.10.4