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

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Date:	Thu, 05 Sep 2013 14:40:44 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	jesse@...ira.com
Cc:	netdev@...r.kernel.org, dev@...nvswitch.org, azhou@...ira.com,
	fengguan.wu@...el.com, geert@...ux-m68k.org
Subject: Re: [PATCH] openvswitch: Fix alignment of struct sw_flow_key.

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.

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