linux-kernel - Re: [PATCH 3/5] ARM: trusted_foundations: do not use naked function

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Date:   Wed, 21 Mar 2018 12:13:58 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Stefan Agner <stefan@...er.ch>, linux@...linux.org.uk,
        ard.biesheuvel@...aro.org, arnd@...db.de
Cc:     nicolas.pitre@...aro.org, keescook@...omium.org,
        marc.zyngier@....com, linux-kernel@...r.kernel.org,
        mka@...omium.org, linux-arm-kernel@...ts.infradead.org,
        Bernhard.Rosenkranzer@...aro.org
Subject: Re: [PATCH 3/5] ARM: trusted_foundations: do not use naked function

On 20/03/18 23:02, 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.
> 
> Additionally also make sure all callee-saved registers get saved
> as it has been done before.
> 
> Signed-off-by: Stefan Agner <stefan@...er.ch>
> ---
>   arch/arm/firmware/trusted_foundations.c | 12 +++++++-----
>   1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm/firmware/trusted_foundations.c b/arch/arm/firmware/trusted_foundations.c
> index 3fb1b5a1dce9..426d732e6591 100644
> --- a/arch/arm/firmware/trusted_foundations.c
> +++ b/arch/arm/firmware/trusted_foundations.c
> @@ -31,21 +31,23 @@
>   
>   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"
>   		__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}"
>   		:
> -		: "r" (type), "r" (arg1), "r" (arg2)
> -		: "memory");
> +		: "r" (r0), "r" (r1), "r" (r2)
> +		: "memory", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10");

I may be missing a subtlety, but it looks like we no longer have a 
guarantee that r11 will be caller-saved as it was previously. I don't 
know the Trusted Foundations ABI to say whether that matters or not, but 
if it is the case that it never needed preserving anyway, that might be 
worth calling out in the commit message.

Robin.

>   }
>   
>   static int tf_set_cpu_boot_addr(int cpu, unsigned long boot_addr)
>