linux-kernel - Re: [PATCH v2 03/44] Add CONFIG_HAVE_64BIT_ALIGNED

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Date:	Mon, 17 Dec 2012 11:11:30 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	james.hogan@...tec.com
Cc:	geert@...ux-m68k.org, hpa@...or.com, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, arnd@...db.de, bsingharora@...il.com,
	mingo@...nel.org, akpm@...ux-foundation.org, eparis@...hat.com,
	wad@...omium.org, james.l.morris@...cle.com, netdev@...r.kernel.org
Subject: Re: [PATCH v2 03/44] Add CONFIG_HAVE_64BIT_ALIGNED_STRUCT for
 taskstats

From: James Hogan <james.hogan@...tec.com>
Date: Mon, 17 Dec 2012 09:51:04 +0000

> On 10/12/12 12:55, Geert Uytterhoeven wrote:
>> On Mon, Dec 10, 2012 at 11:22 AM, James Hogan <james.hogan@...tec.com> wrote:
>>> On 08/12/12 03:43, H. Peter Anvin wrote:
>>>> On 12/05/2012 08:08 AM, James Hogan wrote:
>>>>> On 64 bit architectures with no efficient unaligned access, taskstats
>>>>> has to add some padding to a reply to prevent unaligned access warnings.
>>>>> However this also needs to apply to 32 bit architectures with 64 bit
>>>>> struct alignment such as metag (which has 64 bit memory accesses).
>>>>
>>>> Wait... 64-bit struct alignment on structures with only 32-bit members?
>>>>  That might be... interesting... in a number of places...
>>>
>>> I'll rewrite the description as it's a bit misleading. On metag 64bit
>>> struct alignment is required when it contains 64bit members, not if it
>>> only contains 32bit members. Although metag is a 32bit arch, it can do
>>> 64bit memory accesses which must be aligned.
>> 
>> The C alignment rules should take care of this automatically (struct alignment
>> is the maximum alignment of its members).
> 
> Hi Geert,
> 
> Please see the comment in mk_reply in kernel/taskstats.c. The structure
> is being serialised after 2 NLA headers and a pid which is why the extra
> padding needs to be added manually.

Please solve netlink attribute 64-bit alignment issues fundamentally
rather on a case-by-case basis.

We were just discussing this very issue on the netdev list recently,
CC:'d
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