Context switching through an ISR in RTEMS

[Utkarsh Rai]

On Sat, Aug 15, 2020 at 7:26 PM Gedare Bloom <gedare at rtems.org> wrote:

> On Sat, Aug 15, 2020 at 6:26 AM Utkarsh Rai <utkarsh.rai60 at gmail.com>
> wrote:
> >
> >
> > On Thu, Aug 13, 2020 at 5:10 AM Utkarsh Rai <utkarsh.rai60 at gmail.com>
> wrote:
> >>
> >> Thanks, I'll check them out.
> >>
> >> On Thu, Aug 13, 2020 at 12:56 AM Gedare Bloom <gedare at rtems.org> wrote:
> >>>
> >>> On Wed, Aug 12, 2020 at 11:33 AM Utkarsh Rai <utkarsh.rai60 at gmail.com>
> wrote:
> >>> >
> >>> > Hello,
> >>> > I have been testing my code for thread stack isolation against
> various tests( Some written by me, and remaining already present). One of
> the limitations that I have found is that I encounter fatal errors whenever
> a context switch takes place through an ISR. Can you please explain how the
> context switching procedure works when an interrupt occurs. When I use gdb
> for stepping through the code it asynchronously moves to context switching
> code from the executing thread( for example psx16 test).
> >>> > For thread stack protection,  the part that deals with context
> switching simply 'sets 'the memory entries of the heir stack and 'unsets'
> that of the executing stack.
> >>>
> >>> There are two issues to start: interrupt stacks and dispatching from
> an ISR.
> >>>
> >>> I think you can start by reading some of the documentation:
> >>>
> https://docs.rtems.org/branches/master/c-user/interrupt_manager.html#processing-an-interrupt
> >>>
> >>>
> https://docs.rtems.org/branches/master/c-user/scheduling_concepts.html#dispatching-tasks
> >>>
> >>>
> https://docs.rtems.org/branches/master/c-user/config/general.html#configure-interrupt-stack-size
> >>>
> >>>
> https://docs.rtems.org/branches/master/cpu-supplement/port.html#interrupt-processing
> >>>
> >>> You can also find some material in rtems-docs.git/porting -- I don't
> >>> know where that gets generated.
> >>>
> >>> Continue to ask questions, and writing blog posts.
> >
> >
> > So after going through the materials, I was able to understand how an
> ISR is registered, ISR stack initialization. What is still not clear to me
> is what are the differences between dispatching a task in ISR different
> and  a normal context-switch?
> >
> > For example the psxsignal06 test, we wait for a signal here,  on setting
> the breakpoint at the context switch code (cpu_asm.S), after this line,  I
> find that the heir context stack is the ISR stack. The next thread is
> dispatched from this ISR but as soon as I unset the memory attributes of
> the ISR stack I get a fatal error. One possible reason is that the ISR
> stack is not page aligned and unsettling its attributes unsets nearby
> memory regions. Is there something else that I am missing?
> >
> what else is on the same page as the ISR stack?
>
>
The idle thread stack is between 0x202e40 to 0x203e40 and the ISR stack is
between 0x203e40 to 0x204e40. So when we unset the memory for the ISR it
unsets between 0x203000 to 0x205000, I think this may be the problem.



> Not quite related, you'll need to also make sure to map the ISR stack
> back in during ISR Handling, before using it.
>

When the ISR gets called for the first time, it already has R/W permission
and for subsequent context switches it's memory entry is accordingly
set/unset.
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