linux-kernel - Re: [PATCH v2 1/5] arm64: alternative: Provide if/else/endif assembler macros

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Date:	Mon, 20 Jul 2015 18:12:42 +0100
From:	Will Deacon <will.deacon@....com>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Andre Przywara <Andre.Przywara@....com>
Subject: Re: [PATCH v2 1/5] arm64: alternative: Provide if/else/endif
 assembler macros

Hi Daniel,

Couple of really small comments.

On Mon, Jul 20, 2015 at 04:10:00PM +0100, Daniel Thompson wrote:
> The existing alternative_insn macro has some limitations that make it
> hard to work with. In particular the fact it takes instructions from it
> own macro arguments means it doesn't play very nicely with C pre-processor
> macros because the macro arguments look like a string to the C
> pre-processor. Workarounds are (probably) possible but things start to
> look ugly.
> 
> Introduce an alternative set of macros that allows instructions to be
> presented to the assembler as normal and switch everything over to the
> new macros.
> 
> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> ---
>  arch/arm64/include/asm/alternative.h | 40 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/alternative.h b/arch/arm64/include/asm/alternative.h
> index c385a0c4057f..31b19ad18f7e 100644
> --- a/arch/arm64/include/asm/alternative.h
> +++ b/arch/arm64/include/asm/alternative.h
> @@ -77,6 +77,46 @@ void free_alternatives_memory(void);
>  	.org	. - (662b-661b) + (664b-663b)
>  .endm
>  
> +/*
> + * Begin an alternative code sequence.
> + *
> + * The code that follows this marco will be assembled and linked as
> + * normal. There are no restrictions on this code.

s/marco/macro/

> + */
> +.macro alternative_if_not cap
> +	.pushsection .altinstructions, "a"
> +	altinstruction_entry 661f, 663f, \cap, 662f-661f, 664f-663f
> +	.popsection
> +661:
> +.endm
> +
> +/*
> + * Provide the alternative code sequence.
> + *
> + * The code that follows this macro is assembled into a special
> + * section to be used for dynamic patching. Code that follows this
> + * macro must:
> + *
> + * 1. Be exactly the same length (in bytes) as the default code
> + *    sequence.
> + *
> + * 2. Not jump to local labels defined outside of the alternative
> + *    sequence.

Actually, we fix up the branch target during patching. What you can't do
is jump into *another* alternative sequence.

Will
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