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

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Message-ID: <CAEP_g=8gcCtkv=no=HkVmS+NDWPM-Uu-SVyR+vb-YQ=7TWJsXA@mail.gmail.com>
Date:	Thu, 5 Sep 2013 11:36:19 -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>, fengguan.wu@...el.com,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH] openvswitch: Fix alignment of struct sw_flow_key.

On Thu, Sep 5, 2013 at 11:17 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.
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