Project for GSoC 2020

[Utkarsh Rai]

 Before specifying the interface I want to take your kind feedback on what
all things would need to be done in the interface.

 By looking into the MMU description of ARM and x86, according to me, the
implementation would have to be done in two levels-

1. The architecture-specific part, wherein I will have to initialize the
MMU(already implemented for ARM), TLB management and exception handling.
2. The generalization part, wherein I will need to manage the thread
stacks, their access permissions. Each stack should have a data structure
associated with it that has the permission flags, starting address and
other relevant information. For increased robustness of the thread stacks,
every time a context switch takes place we will have to set up the page
tables, TLBs to map the address of the executing thread as well as that of
the threads it has the permission to access.




On Mon, Mar 9, 2020 at 1:47 AM <dufault at hda.com> wrote:

>
>
> > On Mar 8, 2020, at 15:28 , dufault at hda.com wrote:
> >
> > I have not gone through this thread as thoroughly as I should.  Yet, I
> will still jump in.
> >
> > The stack protectin mechanism should be done in a POSIX compliant way
> without defining any extensions.
> >
> > When threads have protected stack areas that must be shared those
> regions should be shared using an "mmap" interface. That is POSIX
> compliant, and will require extra effort to establish the sharing of
> stack-based memory regions.
> >
> > My two minute analysis of "thread pools" is that protected stacks in
> RTEMS should be implemented without that.  Applications that want protected
> stacks should have the discipline of explicitly requesting access to
> regions of memory that are in another threads stack.  The value of
> protected stacks will not be cost free.
> >
> > I'd like to see:
> >
> > - The ability of each thread to specify a protected stack.
> > - A protected thread's stack memory region shall be unavailable to other
> threads unless it is in a shared memory region.
> > -- I won't define the granularity of what the previous point means.  In
> most implementations, it probably means that if "task2" has access to
> memory in the "task1" stack via mmap then it can probably stomp all over
> any part of task1s stack, it probably implies that the task1 stack memory
> is left accessible when task2 is running instead of being unmapped.
>
> In the Olden Days I worked on an architecture (Alliant mini
> supercomputers) where the stack wasn't in shared memory.  It took a little
> bit of getting used to, but not much.  In that architecture you just
> couldn't share the stack, no hope to "mmap" anything.
>
> We should define the goal of protected stacks.  My goal is increased
> robustness at the expense of additional application setup complexity, but
> the application setup should be 100% POSIX compliant.
>
> Peter
> -----------------
> Peter Dufault
> HD Associates, Inc.      Software and System Engineering
>
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>
>
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