linux-kernel - Re: [PATCH v2 02/17] arm64/lib: copy_page: avoid x18 register in assembler code

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Message-ID: <20191025094137.GB40270@lakrids.cambridge.arm.com>
Date:   Fri, 25 Oct 2019 10:41:37 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     samitolvanen@...gle.com
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 02/17] arm64/lib: copy_page: avoid x18 register in
 assembler code

On Thu, Oct 24, 2019 at 03:51:17PM -0700, samitolvanen@...gle.com wrote:
> From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> 
> Register x18 will no longer be used as a caller save register in the
> future, so stop using it in the copy_page() code.
> 
> Link: https://patchwork.kernel.org/patch/9836869/
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  arch/arm64/lib/copy_page.S | 38 +++++++++++++++++++-------------------
>  1 file changed, 19 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/arm64/lib/copy_page.S b/arch/arm64/lib/copy_page.S
> index bbb8562396af..8b562264c165 100644
> --- a/arch/arm64/lib/copy_page.S
> +++ b/arch/arm64/lib/copy_page.S
> @@ -34,45 +34,45 @@ alternative_else_nop_endif
>  	ldp	x14, x15, [x1, #96]
>  	ldp	x16, x17, [x1, #112]
>  
> -	mov	x18, #(PAGE_SIZE - 128)
> +	add	x0, x0, #256
>  	add	x1, x1, #128
>  1:
> -	subs	x18, x18, #128
> +	tst	x0, #(PAGE_SIZE - 1)
>  
>  alternative_if ARM64_HAS_NO_HW_PREFETCH
>  	prfm	pldl1strm, [x1, #384]
>  alternative_else_nop_endif
>  
> -	stnp	x2, x3, [x0]
> +	stnp	x2, x3, [x0, #-256]
>  	ldp	x2, x3, [x1]
> -	stnp	x4, x5, [x0, #16]
> +	stnp	x4, x5, [x0, #-240]
>  	ldp	x4, x5, [x1, #16]

For legibility, could we make the offset and bias explicit in the STNPs
so that these line up? e.g.

	stnp	x4, x5, [x0, #16 - 256]
	ldp	x4, x5, [x1, #16]

... that'd make it much easier to see by eye that this is sound, much as
I trust my mental arithmetic. ;)

> -	stnp	x6, x7, [x0, #32]
> +	stnp	x6, x7, [x0, #-224]
>  	ldp	x6, x7, [x1, #32]
> -	stnp	x8, x9, [x0, #48]
> +	stnp	x8, x9, [x0, #-208]
>  	ldp	x8, x9, [x1, #48]
> -	stnp	x10, x11, [x0, #64]
> +	stnp	x10, x11, [x0, #-192]
>  	ldp	x10, x11, [x1, #64]
> -	stnp	x12, x13, [x0, #80]
> +	stnp	x12, x13, [x0, #-176]
>  	ldp	x12, x13, [x1, #80]
> -	stnp	x14, x15, [x0, #96]
> +	stnp	x14, x15, [x0, #-160]
>  	ldp	x14, x15, [x1, #96]
> -	stnp	x16, x17, [x0, #112]
> +	stnp	x16, x17, [x0, #-144]
>  	ldp	x16, x17, [x1, #112]
>  
>  	add	x0, x0, #128
>  	add	x1, x1, #128
>  
> -	b.gt	1b
> +	b.ne	1b
>  
> -	stnp	x2, x3, [x0]
> -	stnp	x4, x5, [x0, #16]
> -	stnp	x6, x7, [x0, #32]
> -	stnp	x8, x9, [x0, #48]
> -	stnp	x10, x11, [x0, #64]
> -	stnp	x12, x13, [x0, #80]
> -	stnp	x14, x15, [x0, #96]
> -	stnp	x16, x17, [x0, #112]
> +	stnp	x2, x3, [x0, #-256]
> +	stnp	x4, x5, [x0, #-240]
> +	stnp	x6, x7, [x0, #-224]
> +	stnp	x8, x9, [x0, #-208]
> +	stnp	x10, x11, [x0, #-192]
> +	stnp	x12, x13, [x0, #-176]
> +	stnp	x14, x15, [x0, #-160]
> +	stnp	x16, x17, [x0, #-144]

... likewise here:

	stnp	xt1, xt2, [x0, #off - 256]

I don't see a nicer way to write this sequence without using an
additional register, so with those changes:

Reviewed-by: Mark Rutland <mark.rutland@....com>

Thanks,
Mark.

>  
>  	ret
>  ENDPROC(copy_page)
> -- 
> 2.24.0.rc0.303.g954a862665-goog
>