Xen devel - Jul 2006 - e1000: Problem with "disable CRC stripping workaround" patch

Hi Jesse,
	I just came across this:

  http://www.mail-archive.com/netdev@vger.kernel.org/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.


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On Thu, Jul 20, 2006 at 09:42:48AM -0700, Auke Kok
wrote:
>
> Please give it a try and let us know if it fixes the issue for you, it 
> should be much better than patching xen''s code.
I don''t really care on way or another whether we get the FCS.
However, the important thing to note here for Xen is that MTU
!= MRU.  Just because you''ve set the MTU to 1500 does not imply
that you won''t receive packets > 1500.

Cheers,
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Hi,

On Thu, 2006-07-20 at 09:42 -0700, Auke Kok wrote:
> Mark McLoughlin wrote:
> > Hi Jesse,
> > 	I just came across this:
> > 
> >   http://www.mail-archive.com/netdev@vger.kernel.org/msg14547.html
> > 
> > 	I''m seeing a problem with this currently under
Xen''s bridging
> > configuration.
>  > 	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.
> 
> We were sort of expecting this and sent the following patch upstream
already -
> it''s already queued for 2.6.18-rc3:
> 
>
http://kernel.org/git/?p=linux/kernel/git/jgarzik/netdev-2.6.git;a=commitdiff_plain;h=f235a2abb27b9396d2108dd2987fb8262cb508a3;hp=d3d9e484b2ca502c87156b69fa6b8f8fd5fa18a0
> 
> Please give it a try and let us know if it fixes the issue for you, it
should
> be much better than patching xen''s code.
	Yep, this fixes the problem for me.

Thanks,
Mark.


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