Subject: Re: [PATCH net] net: Allow flow dissector to handle non 4-byte aligned headers

From: Florian Fainelli <[email-masked]>
Date: Sun, 31 Jan 2016 16:39:35 -0800

Le 31/01/2016 16:24, Eric Dumazet a écrit :
> On Sun, 2016-01-31 at 13:37 -0800, Tom Herbert wrote:
>> Call get_unaligned_be32 when we access 32-bit fields in
>> __skb_flow_dissect. At the beginning check for unlikely case of
>> 1-byte aligned packet.
>>
>> Note that flow_dissector may be asked to parse packet unaligned
>> fields in two instances:
>>
>> 1) Packet from a driver which is aligned to Ethernet header
>>    (2-byte alignment)
>> 2) Parsing inner headers of a received GRE-TEB packet
>>
>> Testing: Ran super_netperf tests did not see a regression. This was on
>> x86 which does not have problems with unaligned data.
> 
> But this test is absolutely useless, what about testing arches that
> actually care ?
> 
> I am told all these MIPS based boxes have already not enough cpu power.

How about the Cavium OCTEON family and Broadcom/Netlogic XLR/XLP, those
are massively multi-core and MIPS64 capable, even though they may not
always run a Linux networking stack, some do.

There are also plenty of ARMv7/ARMv8 devices out there that would
benefit from proper alignment some might end-up using mlx4/5 and intel
cards.

> 
> It is sad, because none of them use the drivers that might call flow
> dissection (mlx4 and 4 intel drivers)
> 
> So I would rather fix the cases where flow dissection called from
> eth_get_headlen() with non aligned stuff.
> 
> And maybe restrict GRE-TEB to platforms with
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y, since otherwise we need to add
> these unaligned macros in thousands of places in our stacks.
> 
> 
> 

-- 
Florian