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

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Message-ID: <20150716181929.GU26390@arm.com>
Date:	Thu, 16 Jul 2015 19:19:29 +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] arm64: alternative: Provide if/else/endif assembler
 macros

Hi Daniel,

On Fri, Jul 10, 2015 at 02:48:50PM +0100, Daniel Thompson wrote:
> The existing alternative_insn macro has some limitations that make it
> hard to work with. In partiuclar 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>
> ---
> 
> Notes:
>     To be honest these if not/else/endif macros are simply more readable
>     than the original macro and that might be enough to justify them on
>     their own. However below is an example that is needlessly hard to
>     write without them because ICC_PMR_EL1 is a C pre-processor macro.
>     
>      	.macro	disable_irq, tmp
>      	mov	\tmp, #ICC_PMR_EL1_MASKED
>     alternative_if_not ARM64_HAS_SYSREG_GIC_CPUIF
>     	msr	daifset, #2
>     alternative_else
>      	msr_s	ICC_PMR_EL1, \tmp
>     alternative_endif
>     	.endm
>     
>     The new macros have received a fair degree of testing because I have
>     based my (not published since March) pseudo-NMI patch set on them.
> 
>  arch/arm64/include/asm/alternative.h | 18 ++++++++++++------
>  arch/arm64/kernel/entry.S            | 29 +++++++++++++----------------
>  arch/arm64/kvm/hyp.S                 | 12 ++++++++++--
>  arch/arm64/mm/cache.S                |  7 ++++++-
>  4 files changed, 41 insertions(+), 25 deletions(-)

After some consideration, I think I prefer your suggestion over what we
currently have in mainline. However, there are a bunch of patches that
are candidates for 4.3 which will conflict horribly with this.

Would you be able to:

  (1) Split this up so that you have a patch introducing the new macro,
      then a patch converting entry.S and cache.S then a separate one
      for kvm/hyp.S?

  (2) Keep alternative_insn around for the moment

  (3) Once the dust has settled for 4.3, we can see how easy the old
      macro is to remove

Sound ok to you?

Cheers,

Will
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