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

[Arun KS]

Hello Rahul,

On Tue, Mar 25, 2014 at 5:01 PM, Rahul Garg <rahul.lnmiit at gmail.com> wrote:
> 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

Its actually SVC mode. not system mode.
arch/arm/kernel/entry-armv.S. This file is your key.

>
> And thanks for your prompt answer :)

My pleasure.

Thanks,
Arun

>
> 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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