linux-kernel - Re: [RESEND PATCH v2] bus: arm-cci: use asm unreachable

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Message-ID: <9925be9a-2e24-7ab4-4d7e-cb8f62c422e7@arm.com>
Date:   Tue, 8 May 2018 13:19:30 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Stefan Agner <stefan@...er.ch>, arm@...nel.org, arnd@...db.de
Cc:     linux@...linux.org.uk, ard.biesheuvel@...aro.org,
        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
Subject: Re: [RESEND PATCH v2] bus: arm-cci: use asm unreachable

On 08/05/18 12:32, Stefan Agner wrote:
> Mixing asm and C code is not recommended in a naked function by
> gcc and leads to an error when using clang:
>    drivers/bus/arm-cci.c:2107:2: error: non-ASM statement in naked
>    function is not supported
>          unreachable();
>          ^
> 
> While the function is marked __naked it actually properly return
> in asm. There is no need for the unreachable() call.

The commit title is a bit out-of-date now (I guess it could just be 
something like "remove unnecessary unreachable()" now), but the rest 
looks OK to me - even GCC clearly doesn't expect anything beyond the asm 
to be reachable anyway since the lack of epilogue includes the lack of 
any compiler-generated return. I've checked that GCC 7.2 generates 
identical object files before and after, other than (for obvious 
reasons) the line numbers generated by WANT_WARN_ON_SLOWPATH for all the 
WARN()s appearing later in the file.

Reviewed-by: Robin Murphy <robin.murphy@....com>

> Suggested-by: Russell King <linux@....linux.org.uk>
> Signed-off-by: Stefan Agner <stefan@...er.ch>
> Acked-by: Nicolas Pitre <nico@...aro.org>
> ---
>   drivers/bus/arm-cci.c | 2 --
>   1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/bus/arm-cci.c b/drivers/bus/arm-cci.c
> index 443e4c3fd357..b8184a903583 100644
> --- a/drivers/bus/arm-cci.c
> +++ b/drivers/bus/arm-cci.c
> @@ -371,8 +371,6 @@ asmlinkage void __naked cci_enable_port_for_self(void)
>   	[sizeof_struct_cpu_port] "i" (sizeof(struct cpu_port)),
>   	[sizeof_struct_ace_port] "i" (sizeof(struct cci_ace_port)),
>   	[offsetof_port_phys] "i" (offsetof(struct cci_ace_port, phys)) );
> -
> -	unreachable();
>   }
>   
>   /**
>