Re: [RFC PATCH] Flipped jump labels

From: Borislav Petkov
Date: Sun Aug 10 2014 - 11:35:53 EST

On Sun, Aug 10, 2014 at 08:13:03AM +0200, Ingo Molnar wrote:
> Wouldn't using STATIC_KEY_INIT_TRUE and static_key_true() [instead of
> !static_key_false()] result in the same good code placement effects?

Nope, not really. static_key_true() is !static_key_false() and we're not
changing anything, logically. ASM looks non-optimal here, in any case,
with the "true" version.

Also, the problem in this particular use case in native_sched_clock is
that we want to do some initialization first before we enable the RDTSC,
i.e., the likely branch.

So we want to have the following static layout after build:


and the initialization code turns the JMP into a NOP at runtime.
Basically something like alternatives. :-)

Current jump labels do not have the possibility to get generated as
jump-to-unlikely labels which get changed at init time - they issue
NOPs only.


Sent from a fat crate under my desk. Formatting is fine.
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