[Qemu-devel] [PATCH V2] virtio-net: unbreak any layout

[Jason Wang <jasowang@redhat.com>]

Commit 032a74a1c0fcdd5fd1c69e56126b4c857ee36611
("virtio-net: byteswap virtio-net header") breaks any layout by
requiring out_sg[0].iov_len >= n->guest_hdr_len. Fixing this by
copying header to temporary buffer if swap is needed, and then use
this buffer as part of out_sg.

Fixes 032a74a1c0fcdd5fd1c69e56126b4c857ee36611
("virtio-net: byteswap virtio-net header")
Cc: qemu-stable@nongnu.org
Cc: clg@fr.ibm.com
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
Changes from V1:
- avoid header copying if there's no need to do header swap
- don't write the header back
---
 hw/net/virtio-net.c               | 17 ++++++++++++++---
 include/hw/virtio/virtio-access.h |  9 +++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index e3c2db3..12322bd 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1142,7 +1142,8 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
         ssize_t ret, len;
         unsigned int out_num = elem.out_num;
         struct iovec *out_sg = &elem.out_sg[0];
-        struct iovec sg[VIRTQUEUE_MAX_SIZE];
+        struct iovec sg[VIRTQUEUE_MAX_SIZE], sg2[VIRTQUEUE_MAX_SIZE];
+        struct virtio_net_hdr hdr;
 
         if (out_num < 1) {
             error_report("virtio-net header not in first element");
@@ -1150,11 +1151,21 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
         }
 
         if (n->has_vnet_hdr) {
-            if (out_sg[0].iov_len < n->guest_hdr_len) {
+            if (iov_size(out_sg, out_num) < n->guest_hdr_len) {
                 error_report("virtio-net header incorrect");
                 exit(1);
             }
-            virtio_net_hdr_swap(vdev, (void *) out_sg[0].iov_base);
+            if (virtio_needs_swap(vdev)) {
+                iov_to_buf(out_sg, out_num, 0, &hdr, sizeof(hdr));
+                virtio_net_hdr_swap(vdev, (void *) &hdr);
+                sg2[0].iov_base = &hdr;
+                sg2[0].iov_len = sizeof(hdr);
+                out_num = iov_copy(&sg2[1], ARRAY_SIZE(sg2) - 1,
+                                   out_sg, out_num,
+                                   sizeof(hdr), -1);
+                out_num += 1;
+                out_sg = sg2;
+            }
         }
 
         /*
diff --git a/include/hw/virtio/virtio-access.h b/include/hw/virtio/virtio-access.h
index cee5dd7..1ec1dfd 100644
--- a/include/hw/virtio/virtio-access.h
+++ b/include/hw/virtio/virtio-access.h
@@ -143,6 +143,15 @@ static inline uint64_t virtio_ldq_p(VirtIODevice *vdev, const void *ptr)
     }
 }
 
+static inline bool virtio_needs_swap(VirtIODevice *vdev)
+{
+#ifdef HOST_WORDS_BIGENDIAN
+    return virtio_access_is_big_endian(vdev) ? false : true;
+#else
+    return virtio_access_is_big_endian(vdev) ? true : false;
+#endif
+}
+
 static inline uint16_t virtio_tswap16(VirtIODevice *vdev, uint16_t s)
 {
 #ifdef HOST_WORDS_BIGENDIAN
-- 
2.1.4