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[Qemu-devel] [PATCH 087/108] virtio-net: byteswap virtio-net header

From: Michael Roth
Subject: [Qemu-devel] [PATCH 087/108] virtio-net: byteswap virtio-net header
Date: Wed, 6 Aug 2014 15:39:37 -0500

From: Cédric Le Goater <address@hidden>

TCP connectivity fails when the guest has a different endianness.
The packets are silently dropped on the host by the tap backend
when they are read from user space because the endianness of the
virtio-net header is in the wrong order. These lines may appear
in the guest console:

[  454.709327] skbuff: bad partial csum: csum=8704/4096 len=74
[  455.702554] skbuff: bad partial csum: csum=8704/4096 len=74

The issue that got first spotted with a ppc64le PowerKVM guest,
but it also exists for the less common case of a x86_64 guest run
by a big-endian ppc64 TCG hypervisor.

Signed-off-by: Cédric Le Goater <address@hidden>
[ Ported from PowerKVM,
  Greg Kurz <address@hidden> ]
Signed-off-by: Greg Kurz <address@hidden>
Reviewed-by: Michael S. Tsirkin <address@hidden>
Signed-off-by: Michael S. Tsirkin <address@hidden>

(cherry picked from commit 032a74a1c0fcdd5fd1c69e56126b4c857ee36611)
Signed-off-by: Michael Roth <address@hidden>
 hw/net/virtio-net.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 940a7cf..2ac6ce5 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -863,6 +863,14 @@ static int virtio_net_has_buffers(VirtIONetQueue *q, int 
     return 1;
+static void virtio_net_hdr_swap(struct virtio_net_hdr *hdr)
+    tswap16s(&hdr->hdr_len);
+    tswap16s(&hdr->gso_size);
+    tswap16s(&hdr->csum_start);
+    tswap16s(&hdr->csum_offset);
 /* dhclient uses AF_PACKET but doesn't pass auxdata to the kernel so
  * it never finds out that the packets don't have valid checksums.  This
  * causes dhclient to get upset.  Fedora's carried a patch for ages to
@@ -898,6 +906,7 @@ static void receive_header(VirtIONet *n, const struct iovec 
*iov, int iov_cnt,
         void *wbuf = (void *)buf;
         work_around_broken_dhclient(wbuf, wbuf + n->host_hdr_len,
                                     size - n->host_hdr_len);
+        virtio_net_hdr_swap(wbuf);
         iov_from_buf(iov, iov_cnt, 0, buf, sizeof(struct virtio_net_hdr));
     } else {
         struct virtio_net_hdr hdr = {
@@ -1106,6 +1115,14 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue *q)
+        if (n->has_vnet_hdr) {
+            if (out_sg[0].iov_len < n->guest_hdr_len) {
+                error_report("virtio-net header incorrect");
+                exit(1);
+            }
+            virtio_net_hdr_swap((void *) out_sg[0].iov_base);
+        }
          * If host wants to see the guest header as is, we can
          * pass it on unchanged. Otherwise, copy just the parts

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