[PATCH] virtio_net: Fix MAX_PACKET_LEN to support 802.1Q VLANs

[PATCH] virtio_net: Fix MAX_PACKET_LEN to support 802.1Q VLANs

[Alex Williamson]

802.1Q expanded the maximum ethernet frame size by 4 bytes for the
VLAN tag.  We're not taking this into account in virtio_net, which
means the buffers we provide to the backend in the virtqueue RX ring
aren't big enough to hold a full MTU VLAN packet.  For QEMU/KVM,
this results in the backend exiting with a packet truncation error.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>
---

 drivers/net/virtio_net.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 43f6523..63ef2a8 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -24,6 +24,7 @@
 #include <linux/virtio.h>
 #include <linux/virtio_net.h>
 #include <linux/scatterlist.h>
+#include <linux/if_vlan.h>
 
 static int napi_weight = 128;
 module_param(napi_weight, int, 0444);
@@ -33,7 +34,7 @@ module_param(csum, bool, 0444);
 module_param(gso, bool, 0444);
 
 /* FIXME: MTU in config. */
-#define MAX_PACKET_LEN (ETH_HLEN+ETH_DATA_LEN)
+#define MAX_PACKET_LEN (ETH_HLEN + VLAN_HLEN + ETH_DATA_LEN)
 #define GOOD_COPY_LEN	128
 
 struct virtnet_info

Re: [PATCH] virtio_net: Fix MAX_PACKET_LEN to support 802.1Q VLANs

[Mark McLoughlin]

On Tue, 2009-01-20 at 09:48 -0700, Alex Williamson wrote:

> 802.1Q expanded the maximum ethernet frame size by 4 bytes for the
> VLAN tag.  We're not taking this into account in virtio_net, which
> means the buffers we provide to the backend in the virtqueue RX ring
> aren't big enough to hold a full MTU VLAN packet.  For QEMU/KVM,
> this results in the backend exiting with a packet truncation error.
> 
> Signed-off-by: Alex Williamson <alex.williamson@hp.com>

Acked-by: Mark McLoughlin <markmc@redhat.com>

Note 1: this only matters if the guest isn't using mergeable receive
buffers or GSO

Note 2: IMHO, this should go in 2.6.29 and stable

Cheers,
Mark.

Re: [PATCH] virtio_net: Fix MAX_PACKET_LEN to support 802.1Q VLANs

[David Miller]

From: Mark McLoughlin <markmc@redhat.com>
Date: Tue, 20 Jan 2009 16:59:26 +0000

> On Tue, 2009-01-20 at 09:48 -0700, Alex Williamson wrote:
> 
> > 802.1Q expanded the maximum ethernet frame size by 4 bytes for the
> > VLAN tag.  We're not taking this into account in virtio_net, which
> > means the buffers we provide to the backend in the virtqueue RX ring
> > aren't big enough to hold a full MTU VLAN packet.  For QEMU/KVM,
> > this results in the backend exiting with a packet truncation error.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@hp.com>
> 
> Acked-by: Mark McLoughlin <markmc@redhat.com>
> 
> Note 1: this only matters if the guest isn't using mergeable receive
> buffers or GSO
> 
> Note 2: IMHO, this should go in 2.6.29 and stable

Applied and queued up for -stable, thanks everyone!