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

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Date:	Thu, 10 Mar 2011 13:20:00 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	David Daney <ddaney@...iumnetworks.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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 Thu, Mar 10, 2011 at 01:04:01PM -0500, Steven Rostedt wrote:
> On Thu, 2011-03-10 at 09:27 -0800, David Daney wrote:
> > On 03/10/2011 07:38 AM, Steven Rostedt wrote:
> 
> > >> Can you explain what would prevent gcc from aligning these 3 pointers
> > >> (total of 24 bytes on 64-bit architectures) on 32-bytes ?
> > 
> > I can:
> > 
> > http://www.x86-64.org/documentation/abi.pdf Section 3.1.2:
> > 
> >     Aggregates and Unions
> 
> Note, we are not dealing with C or arrays, but with inline assembly, and
> the linker.
> 
> +static __always_inline bool arch_static_branch(struct jump_label_key *key)
> +{
> +       asm goto("1:\tnop\n\t"
> +               "nop\n\t"
> +               ".pushsection __jump_table,  \"aw\"\n\t"
> +               WORD_INSN " 1b, %l[l_yes], %0\n\t"
> +               ".popsection\n\t"
> +               : :  "i" (key) : : l_yes);
> +       return false;
> +l_yes:
> +       return true;
> +}
> 
> 
> That push/pop section part creates the structure we are talking about.
> It's made up of three pointers. The address of the nop, the address of
> the label l_yes and the address of the key.
> 
> Now its up to the linker to decide where to place that element. Can we
> guarantee that it will always be on an 8 byte boundery?
> Hmm, I wonder if we could add a .ALIGN sizeof(long) before that?
> 

we can. Sparc does it, see: arch/sparc/include/asm/jump_label.h.

So I guess it would be .align 8 for 64-bit and .align 4 for 32-bit...

-Jason
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