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

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Date:   Fri, 09 Sep 2016 15:39:33 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     David Vrabel <david.vrabel@...rix.com>
Cc:     <netdev@...r.kernel.org>, Juergen Gross <jgross@...e.com>,
        Patrick Talbert <ptalbert@...hat.com>,
        <linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH net-next] xen-netfront: avoid packet loss when ethernet header crosses page boundary

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> Vitaly Kuznetsov <vkuznets@...hat.com> writes:
>
>> David Vrabel <david.vrabel@...rix.com> writes:
>>
>>> On 22/08/16 16:42, Vitaly Kuznetsov wrote:
>>>> 
>>>> 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