How Kernel stack is used in case of different processor mode in ARM architecture?

[Rahul Garg]

Hi Arun,

When I used word nested interrupt, I meant that in my interrupt
handler I am enabling interrupts.

and about "what made you believe we need system mode to support nesting?"

I asked this question on SO, here is the link for its answer

http://stackoverflow.com/a/22500017/769260

And thanks for your prompt answer :)

Regards
Rahul

On Tue, Mar 25, 2014 at 4:45 PM, Arun KS <getarunks at gmail.com> wrote:
> On Tue, Mar 25, 2014 at 4:40 PM, Arun KS <getarunks at gmail.com> wrote:
>> Hello Rahul,
>>
>> On Tue, Mar 25, 2014 at 4:10 PM, Rahul Garg <rahul.lnmiit at gmail.com> wrote:
>>> Hi Arun,
>>>
>>> Lines from Robert Love :
>>>
>>> Early in the 2.6 kernel process, an option was added to reduce the
>>> stack size from two
>>> pages down to one, providing only a 4KB stack on 32-bit systems.This
>>> reduced memory
>>> pressure because every process on the system previously needed two
>>> pages of contiguous,
>>> nonswappable kernel memory.To cope with the reduced stack size,
>>> interrupt handlers
>>> were given their own stack, one stack per processor, one page in
>>> size.This stack is referred
>>> to as the interrupt stack.Although the total size of the interrupt
>>> stack is half that of the
>>> original shared stack, the average stack space available is greater
>>> because interrupt handlers
>>> get the full page of memory to themselves.
>>
>> Kernel stack size is architecture dependent. Some architecture uses
>> CONFIG_4KSTACKS to choose 4K stacks.
>>
>> Where as in arm, it is always 8KB.
>>
>> File:arch/arm/include/asm/thread_info.h
>> #define THREAD_SIZE_ORDER       1
>> #define THREAD_SIZE             8192
>> #define THREAD_START_SP         (THREAD_SIZE - 8)
>>
>>>
>>>
>>> So with these lines, it is clear that interrupt stack is used by
>>> interrupt handlers. So can you please re-confirm your answer ?
>> On ARM when there is an irq, the processor switches to irq mode. But
>> the kernel switches to svc mode immediately and uses SVC stack, ie the
>> stack of the current process.
>>
>> Why do you believe my be? :-)
>> If you want re-confirmation, go through arch/arm/kernel/entry-armv.S
>>
>> Thanks,
>> Arun
>>>
>>> And one more thing, as you mentioned only interrupt, undefined and
>>> abort have stack, So how nested interrupt is handled because for that
>>> we need System mode stack ?
> Interrupts are no more nested in linux kernel,
>
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/kernel/irq/handle.c?id=e58aa3d2d0cc01ad8d6f7f640a0670433f794922
>
> But nested exceptions(data, prefetch aborts etc) can still happen.
>
> And, what made you believe we need system mode to support nesting?
> What difference does it make if it is svc mode?
>
> Thanks,
> Arun
>
>
>> ARM linux never use system mode for anything.
>>
>>>
>>> Regards
>>> Rahul
>>>
>>> On Tue, Mar 25, 2014 at 3:06 PM, Arun KS <getarunks at gmail.com> wrote:
>>>> On Tue, Mar 25, 2014 at 2:58 PM, Arun KS <getarunks at gmail.com> wrote:
>>>>> Hello Rahul,
>>>>>
>>>>> On Tue, Mar 25, 2014 at 6:29 AM, Rahul Garg <rahul.lnmiit at gmail.com> wrote:
>>>>>> As I understand every process have a user stack and kernel stack.
>>>>> True.
>>>>>
>>>>>> Apart from that there is a stack for every mode in ARM achitecture. So
>>>>> This is wrong.
>>>>> Only irq, abort and undefined modes have stacks in linux. That too is
>>>>> very limited, 3 bytes per mode per cpu.
>>>>> Have a look at arch/arm/kernel/irq.c
>>>> Sorry. Wrong file, its at arch/arm/kernel/setup.c
>>>>
>>>> Thanks,
>>>> Arun
>>>>> struct stack {
>>>>>         u32 irq[3];
>>>>>         u32 abt[3];
>>>>>         u32 und[3];
>>>>> } ____cacheline_aligned;
>>>>>
>>>>> kernel runs in SVC mode and the stack used belong to the kernel stack
>>>>> of the current task.
>>>>> Even irq, abort and undefined exception handlers use kernel stack of
>>>>> current task. All the exception
>>>>> handlers switch to SVC mode at a very early stage and use kernel
>>>>> stack. Those 3 bytes are used
>>>>> as stack just during the transition phase(for example transition from
>>>>> irq to svc mode during and interrupt).
>>>>>
>>>>> Thanks,
>>>>> Arun
>>>>>> I want to know How different stack and stack pointer works in ARM
>>>>>> modes? Also when this kernel stack associated with the process will be
>>>>>> used ?
>>>>>>
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