netdev - Re: [PATCH net-next V2] net/vxlan: Avoid unaligned access in vxlan_build

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Message-ID: <f0ae4d21-8e72-079a-dfd3-c315f24f0e7c@stressinduktion.org>
Date:   Wed, 21 Sep 2016 18:49:18 +0200
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Sowmini Varadhan <sowmini.varadhan@...cle.com>,
        netdev@...r.kernel.org, jbenc@...hat.com, davem@...emloft.net
Subject: Re: [PATCH net-next V2] net/vxlan: Avoid unaligned access in
 vxlan_build_skb()

On 21.09.2016 18:14, Alexei Starovoitov wrote:
> On Wed, Sep 21, 2016 at 12:10:55PM +0200, Hannes Frederic Sowa wrote:
>> On 20.09.2016 20:57, Sowmini Varadhan wrote:
>>> The vxlan header may not be aligned to 4 bytes in
>>> vxlan_build_skb (e.g., for MLD packets). This patch
>>> avoids unaligned access traps from vxlan_build_skb
>>> (in platforms like sparc) by making struct vxlanhdr __packed.
>>>
>>> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@...cle.com>
>>
>> Performance wise this should only affect code generation for archs where
>> it matters anyway.
> 
> I think it's the opposite. Even on x86 compiler will use byte loads.

I checked that on x86-64 actually. Also clearly visible here:

https://godbolt.org/g/xsW2P1

Bye,
Hannes