netdev - Re: [PATCH] openvswitch: Fix alignment of struct sw_flow

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Date:	Thu, 5 Sep 2013 12:14:08 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	Andy Zhou <azhou@...ira.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] openvswitch: Fix alignment of struct sw_flow_key.

On Thu, Sep 5, 2013 at 11:40 AM, David Miller <davem@...emloft.net> wrote:
> From: Jesse Gross <jesse@...ira.com>
> Date: Thu, 5 Sep 2013 11:36:19 -0700
>
>> On Thu, Sep 5, 2013 at 11:17 AM, David Miller <davem@...emloft.net> wrote:
>>> From: Jesse Gross <jesse@...ira.com>
>>> Date: Thu,  5 Sep 2013 10:41:27 -0700
>>>
>>>> -} __aligned(__alignof__(long));
>>>> +} __aligned(8); /* 8 byte alignment ensures this can be accessed as a long */
>>>
>>> This kind of stuff drives me crazy.
>>>
>>> If the issue is the type, therefore at least use an expression that
>>> mentions the type explicitly.  And mention the actual type that
>>> matters.  "long" isn't it.
>>
>> 'long' actually is the real type here.
>>
>> When doing comparisons, this structure is being accessed as a byte
>> array in 'long' sized chunks, not by its members. Therefore, the
>> compiler's alignment does not necessarily correspond to anything for
>> this purpose. It could be a struct full of u16's and we would still
>> want to access it in chunks of 'long'.
>>
>> To completely honest, I think the correct alignment should be
>> sizeof(long) because I know that 'long' is not always 8 bytes on all
>> architectures. However, you made the point before that this could
>> break the alignment of the 64-bit values on architectures where 'long'
>> is 32 bits wide, so 8 bytes is the generic solution.
>
> Look at net/core/flow.c:flow_key_compare().
>
> And then we annotate struct flowi with
>
> } __attribute__((__aligned__(BITS_PER_LONG/8)));
>
> Don't reinvent the wheel, either mimick how existing code does
> this kind of thing or provide a justification for doing it differently
> and update the existing cases to match and be consistent.

Sure, I'll send a new patch to use BITS_PER_LONG.
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