[Xen-devel] e1000: Problem with "disable CRC stripping workaround" patch

[Mark McLoughlin <markmc@xxxxxxxxxx>]

Hi Jesse,
        I just came across this:

  http://www.mail-archive.com/netdev@xxxxxxxxxxxxxxx/msg14547.html

        I'm seeing a problem with this currently under Xen's bridging
configuration.

        Basically, with the patch, packets are being dropped at this point in
net/bridge/br_forward.c:

---
int br_dev_queue_push_xmit(struct sk_buff *skb)
{
        /* drop mtu oversized packets except gso */
        if (packet_length(skb) > skb->dev->mtu && !skb_is_gso(skb)) {
                kfree_skb(skb);
--

        What's happening that a 1500 byte packet comes in from e1000, onto the
bridge and is to be forwarded to a device whose mtu is 1500. Because the
CRC hasn't been stripped, skb->len is 1504 and the packet is dropped.

        One option is to fix this specific problem is to subtract the CRC
length from skb->len in e1000, another is to raise the MTU on the
receive side of Xen's loopback interface. I've attached a patch for the
latter, but I've no real opinion on which is more correct.

Cheers,
Mark.

xen-netback-jumbo-mtu-on-vif.patch
Description: Text Data

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