linux-kernel - Re: [PATCH 1/2] ARM: copypage-fa: add kto and kfrom to input operands list

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Message-ID: <20181015222302.GZ30658@n2100.armlinux.org.uk>
Date:   Mon, 15 Oct 2018 23:23:03 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Stefan Agner <stefan@...er.ch>
Cc:     ulli.kroll@...glemail.com, joel@....id.au, nico@...aro.org,
        arnd@...db.de, linus.walleij@...aro.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] ARM: copypage-fa: add kto and kfrom to input
 operands list

On Tue, Oct 16, 2018 at 12:16:29AM +0200, Stefan Agner wrote:
> When functions incoming parameters are not in input operands list gcc
> 4.5 does not load the parameters into registers before calling this
> function but the inline assembly assumes valid addresses inside this
> function. This breaks the code because r0 and r1 are invalid when
> execution enters v4wb_copy_user_page ()

NAK.  Naked functions must never be inlined.  Please add a "noinline"
attribute to the function rather than making things more complex.

The GCC manual states:

`naked'
     Use this attribute on the ARM, AVR, MCORE, MSP430, NDS32, RL78, RX
     and SPU ports to indicate that the specified function does not
     need prologue/epilogue sequences generated by the compiler.  It is
     up to the programmer to provide these sequences. The only
                                                      ^^^^^^^^
     statements that can be safely included in naked functions are
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     `asm' statements that do not have operands.  All other statements,
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     including declarations of local variables, `if' statements, and so
     forth, should be avoided.  Naked functions should be used to
     implement the body of an assembly function, while allowing the
     compiler to construct the requisite function declaration for the
     assembler.

The 'I' attribute is fine here because it is a constant that is not
allowed to be in a register (and hence has no code generation side
effects.)

Adding operands for the input parameters, however, isn't going to
work around the fact that _this_ assembly is written to be out of
line and so it must never be inlined by the compiler.

> Also the constant needs to be used as third input operand so account
> for that as well.
> 
> This fixes copypage-fa.c what has previously done before for the other
> copypage implementations in commit 9a40ac86152c ("ARM: 6164/1: Add kto
> and kfrom to input operands list.").
> 
> Signed-off-by: Stefan Agner <stefan@...er.ch>
> ---
>  arch/arm/mm/copypage-fa.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/mm/copypage-fa.c b/arch/arm/mm/copypage-fa.c
> index d130a5ece5d5..ec6501308c60 100644
> --- a/arch/arm/mm/copypage-fa.c
> +++ b/arch/arm/mm/copypage-fa.c
> @@ -22,7 +22,7 @@ fa_copy_user_page(void *kto, const void *kfrom)
>  {
>  	asm("\
>  	stmfd	sp!, {r4, lr}			@ 2\n\
> -	mov	r2, %0				@ 1\n\
> +	mov	r2, %2				@ 1\n\
>  1:	ldmia	r1!, {r3, r4, ip, lr}		@ 4\n\
>  	stmia	r0, {r3, r4, ip, lr}		@ 4\n\
>  	mcr	p15, 0, r0, c7, c14, 1		@ 1   clean and invalidate D line\n\
> @@ -36,7 +36,7 @@ fa_copy_user_page(void *kto, const void *kfrom)
>  	mcr	p15, 0, r2, c7, c10, 4		@ 1   drain WB\n\
>  	ldmfd	sp!, {r4, pc}			@ 3"
>  	:
> -	: "I" (PAGE_SIZE / 32));
> +	: "r" (kto), "r" (kfrom), "I" (PAGE_SIZE / 32));
>  }
>  
>  void fa_copy_user_highpage(struct page *to, struct page *from,
> -- 
> 2.19.1
> 

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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