Subject: Re: [PATCH 0/3] jump label: updates for 2.6.37
From: Steven Rostedt <rostedt@goodmis.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jason Baron <jbaron@redhat.com>, mingo@elte.hu, peterz@infradead.org,
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        fweisbec@gmail.com, avi@redhat.com, davem@davemloft.net,
        sam@ravnborg.org, ddaney@caviumnetworks.com, michael@ellerman.id.au,
        linux-kernel@vger.kernel.org
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Date: Tue, 23 Nov 2010 19:10:16 -0500
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On Tue, 2010-11-23 at 15:32 -0800, H. Peter Anvin wrote:
> On 11/23/2010 03:11 PM, Steven Rostedt wrote:
> >> I would also like to see a change in the API, preferrably something
> >> closer to the "SWITCH_POINT" interface I discussed with Stephen before
> >> Kernel Summit.
> > 
> > Could you explain in more detail what you would like to see.
> 
> The JUMP_LABEL() macro is rather ugly, and I found from the
> static_cpu_has() work that inlines like (somewhat pseudocode here):
> 
> static inline bool SWITCH_POINT(void *metadata)
> {
> 	asm goto("1: <5 byte nop>\n"
> 	         ".section \".metadata\",\"a\"\n"
> 		 ".long 1b, %p0\n"
> 		 ".previous\n"
> 	         : : "i" (metadata)
> 		 : : l_yes);
> 	return false;
> l_yes:
> 	return true;
> }
> 
> ... work quite well; with the resulting SWITCH_POINT() being usable like
> any other boolean expression in the kernel, i.e. as part of if, while,
> etc.  Most of the time, gcc is smart enough to just use the flow of
> control provided, and it also permits backwards compatibility with older
> gcc by patching in a one-byte immediate instead.
> 
> There are some instances where it double-jumps; those can be avoided by
> always jumping (allowing the patch code to replace the jump with a
> 5-byte NOP opportunistically a posteori) but unfortunately current gcc
> tends to not order the sequentially next code afterwards if one does that.
> 

So you would rather have it as an if statement? Something like this:

	if (unlikely(JUMP_LABEL(key)))
		__DO_TRACE(....);

(Note, I like the above better too)

-- Steve


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