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

[Ard Biesheuvel]

On 13 July 2017 at 11:41, Dave Martin <Dave.Martin at arm.com> wrote:
> 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'".
>

Ah ok, so it needs to live outside of the function, just like
current_stack_pointer.

>
> 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).
>

Indeed (assuming you meant mov not mrs)