linux-kernel - Re: [PATCH -v2 6/8] jump_label: Add a new static

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Date:	Wed, 29 Jul 2015 08:43:22 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jasonbaron0@...il.com, bp@...en8.de, luto@...capital.net,
	tglx@...utronix.de, rostedt@...dmis.org, will.deacon@....com,
	liuj97@...il.com, rabin@....in, ralf@...ux-mips.org,
	ddaney@...iumnetworks.com, benh@...nel.crashing.org,
	michael@...erman.id.au, davem@...emloft.net, vbabka@...e.cz
Subject: Re: [PATCH -v2 6/8] jump_label: Add a new static_key interface

On Tue, Jul 28, 2015 at 03:21:01PM +0200, Peter Zijlstra wrote:
> There are various problems and short-comings with the current
> static_key interface:
> 
>  - static_key_{true,false}() read like a branch depending on the key
>    value, instead of the actual likely/unlikely branch depending on
>    init value.
> 
>  - static_key_{true,false}() are, as stated above, tied to the
>    static_key init values STATIC_KEY_INIT_{TRUE,FALSE}.
> 
>  - we're limited to the 2 (out of 4) possible options that compile to
>    a default NOP because that's what our arch_static_branch() assembly
>    emits.
> 
> So provide a new static_key interface:
> 
>   DEFINE_STATIC_KEY_TRUE(name);
>   DEFINE_STATIC_KEY_FALSE(name);
> 
> Which define a key of different types with an initial true/false
> value.
> 
> Then allow:
> 
>    static_branch_likely()
>    static_branch_unlikely()
> 
> to take a key of either type and emit the right instruction for the
> case.
> 
> This means adding a second arch_static_branch_jump() assembly helper
> which emits a JMP per default.
> 
> In order to determine the right instruction for the right state,
> encode the branch type in the LSB of jump_entry::key.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/arm/include/asm/jump_label.h     |   24 +++--
>  arch/arm64/include/asm/jump_label.h   |   18 +++-
>  arch/mips/include/asm/jump_label.h    |   19 ++++
>  arch/powerpc/include/asm/jump_label.h |   19 ++++
>  arch/s390/include/asm/jump_label.h    |   19 ++++
>  arch/sparc/include/asm/jump_label.h   |   35 ++++++--
>  arch/x86/include/asm/jump_label.h     |   21 ++++
>  include/linux/jump_label.h            |  143 ++++++++++++++++++++++++++++++++--
>  kernel/jump_label.c                   |   35 ++++++--
>  9 files changed, 294 insertions(+), 39 deletions(-)

for the s390 part:

Acked-by: Heiko Carstens <heiko.carstens@...ibm.com>

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