linux-kernel - Re: [PATCH v2] arm64: Introduce IRQ stack

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Date:	Fri, 18 Sep 2015 22:02:50 +0900
From:	Jungseok Lee <jungseoklee85@...il.com>
To:	Catalin Marinas <Catalin.Marinas@....com>
Cc:	James Morse <james.morse@....com>,
	Mark Rutland <Mark.Rutland@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"takahiro.akashi@...aro.org" <takahiro.akashi@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] arm64: Introduce IRQ stack

On Sep 18, 2015, at 2:07 AM, Catalin Marinas wrote:
> On Thu, Sep 17, 2015 at 09:36:04PM +0900, Jungseok Lee wrote:
>> On Sep 17, 2015, at 7:33 PM, James Morse wrote:
>>> 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?
> [...]
>> --- a/arch/arm64/include/asm/thread_info.h
>> +++ b/arch/arm64/include/asm/thread_info.h
>> @@ -77,7 +77,7 @@ static inline struct thread_info *current_thread_info(void)
>> 
>> 	asm volatile("mrs %0, sp_el0" : "=r" (sp_el0));
>> 
>> -	return (struct thread_info *)(sp_el0 & ~(THREAD_SIZE - 1));
>> +	return (struct thread_info *)sp_el0;
>> }
> 
> This makes sense, since we just use sp_el0 as a scratch register, store
> the current thread_info address directly. But, as James mentioned, I
> don't think you need asm volatile, just asm (it has a small impact in my
> tests).

I will squash this change into the original one without volatile.

Best Regards
Jungseok Lee--
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