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

[Greg Kurz]

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

This patch adds a flag to inform the vhost kernel backend about the
endianness to use.

This patch is based on previous work by Cédric Le Goater.

Cc: Cédric Le Goater <c...@fr.ibm.com>
Signed-off-by: Greg Kurz <gk...@linux.vnet.ibm.com>
---
 hw/virtio/vhost.c           |   10 +++++++++-
 include/hw/virtio/vhost.h   |    1 +
 linux-headers/linux/vhost.h |    2 ++
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 5a12861..bd442f1 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->legacy_big_endian) {
+       addr.flags |= (1 << VHOST_VRING_F_LEGACY_BIG_ENDIAN);
+    }
+
+    r = dev->vhost_ops->vhost_call(dev, VHOST_SET_VRING_ADDR, &addr);
     if (r < 0) {
         return -errno;
     }
@@ -707,6 +713,8 @@ static int vhost_virtqueue_start(struct vhost_dev *dev,
         goto fail_alloc_ring;
     }
 
+    vq->legacy_big_endian = virtio_is_big_endian(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 d5593d1..991fa20 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 legacy_big_endian; /* Only used with legacy virtio devices */
 };
 
 typedef unsigned long vhost_log_chunk_t;
diff --git a/linux-headers/linux/vhost.h b/linux-headers/linux/vhost.h
index c656f61..ffbb8ee 100644
--- a/linux-headers/linux/vhost.h
+++ b/linux-headers/linux/vhost.h
@@ -34,6 +34,8 @@ struct vhost_vring_addr {
        /* Flag values: */
        /* Whether log address is valid. If set enables logging. */
 #define VHOST_VRING_F_LOG 0
+       /* Whether we have a big-endian legacy virtio device. */
+#define VHOST_VRING_F_LEGACY_BIG_ENDIAN 1
 
        /* Start of array of descriptors (virtually contiguous) */
        __u64 desc_user_addr;