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

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Message-ID: <57BB2E7B.5040303@citrix.com>
Date:   Mon, 22 Aug 2016 17:55:23 +0100
From:   David Vrabel <david.vrabel@...rix.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, <netdev@...r.kernel.org>
CC:     Juergen Gross <jgross@...e.com>,
        Patrick Talbert <ptalbert@...hat.com>,
        <linux-kernel@...r.kernel.org>,
        David Vrabel <david.vrabel@...rix.com>,
        <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

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 (or if necessary, netback could pull a minimum amount).

There's no need to preserve the skb layout (e.g., look how the to-guest
direction we do not do this).

David