linux-kernel - Re: [PATCH 0/2] jump label: update for .39

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Message-ID: <4D795DCE.7050807@caviumnetworks.com>
Date:	Thu, 10 Mar 2011 15:25:02 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Jason Baron <jbaron@...hat.com>, peterz@...radead.org,
	hpa@...or.com, mingo@...e.hu, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, sam@...nborg.org,
	michael@...erman.id.au, linux-kernel@...r.kernel.org,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH 0/2] jump label: update for .39

On 03/10/2011 03:16 PM, Steven Rostedt wrote:
> On Thu, 2011-03-10 at 17:48 -0500, Mathieu Desnoyers wrote:
>> * Steven Rostedt (rostedt@...dmis.org) wrote:
>>> On Thu, 2011-03-10 at 16:22 -0500, Mathieu Desnoyers wrote:
>>>
>>>>> Anyway, I think the best thing for now is to have Jason add
>>>>> the .align(sizeof(long)) in the inline assembly for all locations and be
>>>>> done with it.
>>>>
>>>> You seem to be contradicting yourself here. I'm concerned about having
>>>> "structures" of a size not power of two. Can we simply either
>>>
>>> But we don't have structures. We have data that has been allocated in
>>> assembly. Come to think of it, it may be best to keep these as
>>> ".align 4".
>>
>> The .align 4 is certainly not the right answer, because it will trigger
>> unaligned accesses on some 64-bit architectures, as we have faced with
>> trace event.
>
> Will it? Seems that sparc does this regardless.
>
> Remember, this is 3 natural word sized objects, and vmlinux.ld starts
> the section off with .ALIGN 8, hence, the section is already 8 byte
> aligned, all objects within this section are 8 bytes (for 64bit archs,
> and 4 bytes for 32bit), why would saying ".align 4" cause the linker to
> add holes to make it 4 byte aligned when it is already 8 byte aligned.
> The ".align 4" should work with both 32bit and 64bit archs.
>

It should work, but it hurts my eyes to see the source code forcing a 
64-bit word to 32-bit alignment.

David Daney
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