[RFC PATCH 07/10] arm64: kernel: switch to register x18 as a task struct pointer

[Dave Martin]

On Wed, Jul 12, 2017 at 03:44:20PM +0100, Ard Biesheuvel wrote:
> In order to free up sp_el0, which we will need to deal with faulting
> stack accesses when using virtually mapped stacks, switch to register
> x18 as the task struct register. This is permitted by the AAPCS64 ABI,
> and simplifies many references to 'current', given that they no longer
> involve a MSR instruction to access SP_EL0.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>

[...]

> diff --git a/arch/arm64/include/asm/current.h b/arch/arm64/include/asm/current.h
> index f6580d4afb0e..b4e3acff699c 100644
> --- a/arch/arm64/include/asm/current.h
> +++ b/arch/arm64/include/asm/current.h
> @@ -13,11 +13,9 @@ struct task_struct;
>   */
>  static __always_inline struct task_struct *get_current(void)
>  {
> -	unsigned long sp_el0;
> +	register unsigned long tsk asm ("x18");
>  
> -	asm ("mrs %0, sp_el0" : "=r" (sp_el0));
> -
> -	return (struct task_struct *)sp_el0;
> +	return (struct task_struct *)tsk;

Nit:

You're explicitly returning an uninitialised variable here: the asm
annotation doesn't change the fact that tsk lifetime is that of the
function.   Sufficiently aggressive GCC can probably optimise the whole
thing (and any caller) away as undefined behaviour.

The GCC docs say

"The only supported use for [specifying registers for local variables]
is to specify registers for input and output operands when calling
Extended 'asm'".


As an alternative, you could make tsk a global register variable.  I
don't know whether it should be volatile or not in that case --
probably not, since it's constant for a given thread.

Alternatively, the following should work:

	unsigned long ret;

	asm ("mrs %0, x18" : "=r" (ret));

	return ret;

(with -ffixed-x18, naturally).


[...]

Cheers
---Dave