Re: [Xen-devel] [PATCH net-next v4 2/5] xen-netback: add support for IPv6 checksum offload from guest

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 14/10/13 11:55, Wei Liu wrote:
> On Mon, Oct 14, 2013 at 11:49:20AM +0100, Paul Durrant wrote:
>>> -----Original Message-----
>>> From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
>>> Sent: 14 October 2013 11:43
>>> To: Paul Durrant
>>> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Wei Liu; David Vrabel;
>>> Ian Campbell
>>> Subject: Re: [PATCH net-next v4 2/5] xen-netback: add support for IPv6
>>> checksum offload from guest
>>>
>>> On Fri, Oct 11, 2013 at 04:06:19PM +0100, Paul Durrant wrote:
>>> [...]
>>>> -/*
>>>> - * This is the amount of packet we copy rather than map, so that the
>>>> - * guest can't fiddle with the contents of the headers while we do
>>>> - * packet processing on them (netfilter, routing, etc).
>>>> +/* This is a miniumum size for the linear area to avoid lots of
>>>> + * calls to __pskb_pull_tail() as we set up checksum offsets.
>>>>   */
>>>
>>> You seem to forget to explain why 128 is chosen. :-)
>>
>> Is that not sufficient explanation? What sort of thing are you looking for?
>>
> 
>>From the second version of this patch, we had a conversation.
> 
>> Where does 128 come from?
>>
> 
> "It's just an arbitrary power of 2 that was chosen because it seems to
> cover most likely v6 headers and all v4 headers."
> 
> So something like: "We choose 128 which is likely to cover most V6
> headers and all V4 headers" would be sufficeint.

Is "most IPv6 headers" actually good enough?  Don't we need to ensure
netback copies all IP headers?

David

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