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SubjectRe: [PATCH v2] arm64: Introduce IRQ stack
Hi Will,

On 16/09/15 12:25, Will Deacon wrote:
> On Sun, Sep 13, 2015 at 03:42:17PM +0100, Jungseok Lee wrote:
>> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
>> index dcd06d1..44839c0 100644
>> --- a/arch/arm64/include/asm/thread_info.h
>> +++ b/arch/arm64/include/asm/thread_info.h
>> @@ -73,8 +73,11 @@ static inline struct thread_info *current_thread_info(void) __attribute_const__;
>>
>> static inline struct thread_info *current_thread_info(void)
>> {
>> - return (struct thread_info *)
>> - (current_stack_pointer & ~(THREAD_SIZE - 1));
>> + unsigned long sp_el0;
>> +
>> + asm volatile("mrs %0, sp_el0" : "=r" (sp_el0));
>> +
>> + return (struct thread_info *)(sp_el0 & ~(THREAD_SIZE - 1));
>
> This looks like it will generate worse code than our current implementation,
> thanks to the asm volatile. Maybe just add something like a global
> current_stack_pointer_el0?

Like current_stack_pointer does?:
> register unsigned long current_stack_pointer_el0 asm ("sp_el0");

Unfortunately the compiler won't accept this, as it doesn't like the
register name, it also won't accept instructions in this asm string.

Dropping the 'volatile' has the desired affect[0]. This would only cause a
problem over a call to cpu_switch_to(), which writes to sp_el0, but also
save/restores the callee-saved registers, so they will always be consistent.


James




[0] A fictitious example printk:
> printk("%p%p%u%p", get_fs(), current_thread_info(),
> smp_processor_id(), current);

With this patch compiles to:
5f8: d5384101 mrs x1, sp_el0
5fc: d5384100 mrs x0, sp_el0
600: d5384103 mrs x3, sp_el0
604: d5384104 mrs x4, sp_el0
608: 9272c484 and x4, x4, #0xffffffffffffc000
60c: 9272c463 and x3, x3, #0xffffffffffffc000
610: 9272c421 and x1, x1, #0xffffffffffffc000
614: aa0403e2 mov x2, x4
618: 90000000 adrp x0, 0 <do_bad>
61c: f9400884 ldr x4, [x4,#16]
620: 91000000 add x0, x0, #0x0
624: b9401c63 ldr w3, [x3,#28]
628: f9400421 ldr x1, [x1,#8]
62c: 94000000 bl 0 <printk>

Removing the volatile:
5e4: d5384102 mrs x2, sp_el0
5e8: f9400844 ldr x4, [x2,#16]
5ec: 91000000 add x0, x0, #0x0
5f0: b9401c43 ldr w3, [x2,#28]
5f4: f9400441 ldr x1, [x2,#8]
5f8: 94000000 bl 0 <printk>




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