Re: [Xen-devel] [PATCH net-next] xen-netfront: avoid packet loss when ethernet header crosses page boundary

Vitaly Kuznetsov [off-list ref] writes:

Vitaly Kuznetsov [off-list ref] writes:

quoted

David Vrabel [off-list ref] writes:

quoted

On 22/08/16 16:42, Vitaly Kuznetsov wrote:

quoted

I see two ways to fix the issue:
- Change the 'wire' protocol between netfront and netback to start keeping
  the original SKB structure. We'll have to add a flag indicating the fact
  that the particular request is a part of the original linear part and not
  a frag. We'll need to know the length of the linear part to pre-allocate
  memory.

I don't think there needs to be a protocol change.  I think the check in
netback is bogus -- it's the total packet length that must be >
HLEN_ETH.  The upper layers will pull any headers from the frags as
needed

I'm afraid this is not always true, just removing the check leads us to
the following:

[  495.442186] kernel BUG at ./include/linux/skbuff.h:1927! 
[  495.468789] invalid opcode: 0000 [#1] SMP 

What I wanted to say here is that this test makes me think the
description of the patch I suggested is correct: an SKB can't have its
linear part shorter than ETH_HLEN as the header is being pointed directly,
upper network layers don't assemble it from frags, the check in netback
is valid.

So, how can we proceed here?

Sorry for the second ping but I'd really like to see this moving
forward...

-- 
  Vitaly