linux-kernel - Re: [RFC PATCH] Flipped jump labels

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Date:	Mon, 11 Aug 2014 10:42:58 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Jason Baron <jbaron@...mai.com>
Cc:	Ingo Molnar <mingo@...nel.org>, x86-ml <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] Flipped jump labels

On Sun, Aug 10, 2014 at 11:32:31PM -0400, Jason Baron wrote:
> That is correct. We don't currently support having the default
> branch direction or the fast path be different from how the
> 'static_key' is initialized.
> 
> If I understand your use-case correctly,  you can't have the tsc
> path be the default *before* tsc_init() is called?

Yes. And also have it come first in the function so that after nopping
out the JMP later turns it into a 0-cost path.

> If not, another thought on the implementation here might be
> to re-visit the work the Steven Rostedt  proposed a while back,
> to use jump instead of no-ops by default, and then convert
> the jumps to no-ops in a post-processing phase in order to
> reduce code size (by having 2-byte jumps for example on
> x86). Potentially, we could then avoid converting some of the
> jumps, if they didn't match the default branch direction.
> See: http://www.serverphorums.com/read.php?12,759207

Haha, so I did a very similar thing too, independently from Steve (patch
at the end):

https://lkml.kernel.org/r/20140809105742.GA5910@pd.tnic

but mine is dumber.

I'll dig out Steve's version and play with it a bit.

> In that way there is no API change, we are just relaxing the
> restriction that the default branch direction must match the
> way that the keys are initialized.

Which would make the change even smaller and invisible to users, good!

Thanks.
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