linux-kernel - Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked function

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Date:   Thu, 12 Jul 2018 23:59:33 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Stefan Agner <stefan@...er.ch>
Cc:     ard.biesheuvel@...aro.org, arnd@...db.de, robin.murphy@....com,
        nicolas.pitre@...aro.org, marc.zyngier@....com,
        behanw@...verseincode.com, keescook@...omium.org,
        Bernhard.Rosenkranzer@...aro.org, mka@...omium.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Dmitry Osipenko <digetx@...il.com>,
        Stephen Warren <swarren@...dia.com>,
        Thierry Reding <treding@...dia.com>
Subject: Re: [PATCH v2 3/6] ARM: trusted_foundations: do not use naked
 function

On Sun, Mar 25, 2018 at 08:09:56PM +0200, Stefan Agner wrote:
> As documented in GCC naked functions should only use Basic asm
> syntax. The Extended asm or mixture of Basic asm and "C" code is
> not guaranteed. Currently this works because it was hard coded
> to follow and check GCC behavior for arguments and register
> placement.
> 
> Furthermore with clang using parameters in Extended asm in a
> naked function is not supported:
>   arch/arm/firmware/trusted_foundations.c:47:10: error: parameter
>           references not allowed in naked functions
>                 : "r" (type), "r" (arg1), "r" (arg2)
>                        ^
> 
> Use a regular function to be more portable. This aligns also with
> the other smc call implementations e.g. in qcom_scm-32.c and
> bcm_kona_smc.c.
> 
> Cc: Dmitry Osipenko <digetx@...il.com>
> Cc: Stephen Warren <swarren@...dia.com>
> Cc: Thierry Reding <treding@...dia.com>
> Signed-off-by: Stefan Agner <stefan@...er.ch>
> ---
> Changes in v2:
> - Keep stmfd/ldmfd to avoid potential ABI issues
> 
>  arch/arm/firmware/trusted_foundations.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm/firmware/trusted_foundations.c b/arch/arm/firmware/trusted_foundations.c
> index 3fb1b5a1dce9..689e6565abfc 100644
> --- a/arch/arm/firmware/trusted_foundations.c
> +++ b/arch/arm/firmware/trusted_foundations.c
> @@ -31,21 +31,25 @@
>  
>  static unsigned long cpu_boot_addr;
>  
> -static void __naked tf_generic_smc(u32 type, u32 arg1, u32 arg2)
> +static void tf_generic_smc(u32 type, u32 arg1, u32 arg2)
>  {
> +	register u32 r0 asm("r0") = type;
> +	register u32 r1 asm("r1") = arg1;
> +	register u32 r2 asm("r2") = arg2;
> +
>  	asm volatile(
>  		".arch_extension	sec\n\t"
> -		"stmfd	sp!, {r4 - r11, lr}\n\t"
> +		"stmfd	sp!, {r4 - r11}\n\t"
>  		__asmeq("%0", "r0")
>  		__asmeq("%1", "r1")
>  		__asmeq("%2", "r2")
>  		"mov	r3, #0\n\t"
>  		"mov	r4, #0\n\t"
>  		"smc	#0\n\t"
> -		"ldmfd	sp!, {r4 - r11, pc}"
> +		"ldmfd	sp!, {r4 - r11}\n\t"
>  		:
> -		: "r" (type), "r" (arg1), "r" (arg2)
> -		: "memory");
> +		: "r" (r0), "r" (r1), "r" (r2)
> +		: "memory", "r3", "r12", "lr");
>  }

Does GCC try to inline this?

It may just be better to switch to basic asm.  We know that a naked
function won't be inlined, and we already know (because we need the
prologue/epilogue) what registers the 32-bit arguments will be in.

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