On Thu, Jul 9, 2020 at 8:57 PM Gedare Bloom <ged...@rtems.org> wrote:
> On Thu, Jul 9, 2020 at 9:24 AM Gedare Bloom <ged...@rtems.org> wrote: > > > > On Wed, Jul 8, 2020 at 10:08 PM Utkarsh Rai <utkarsh.ra...@gmail.com> > wrote: > > > > > > > > > > > > > > > On Wed, Jul 8, 2020 at 6:56 PM Gedare Bloom <ged...@rtems.org> wrote: > > >> > > >> On Wed, Jul 8, 2020 at 6:53 AM Sebastian Huber > > >> <sebastian.hu...@embedded-brains.de> wrote: > > >> > > > >> > On 08/07/2020 14:43, Utkarsh Rai wrote: > > >> > > > >> > > Hello, > > >> > > For my GSoC project, I have to provide high-level APIs for sharing > > >> > > isolated stacks. > > >> > > The POSIX compliant high-level way of sharing stacks can be to > create > > >> > > a shared memory object of the stack to be shared through shm_open > and > > >> > > then mmap that to the address space of the current stack. My > doubt is, > > >> > > shm_open() takes the path-name of the shared memory object. Since > this > > >> > > is a high-level API, how does the user 'convert' the stack > address to > > >> > > a shared memory object name? > > >> > Do we need any POSIX compatibility for this? What would you do in a > > >> > POSIX environment? You first get some memory, then hand it over to > > >> > shm_open() to get a file descriptor, then use the file descriptor in > > >> > mmap(), then use this for pthread_attr_setstack() and whatever? > > >> > > >> Yes, but the way to name objects is not set by posix. > > >> > > >> We need to provide our own way of translating an address into a name. > > >> > > >> > > > > >> > > Dr.Gedare mentioned that one way to deal with naming would be > > >> > > something like Mr.Sebastian has been doing with specifications. > From > > >> > > what I could gather, it is a hierarchical way of representing > > >> > > objects(Though, I am not very sure if I understand this > accurately). > > >> > > How can something like this be implemented for naming > stack-addresses? > > >> > I am not sure if the specification of RTEMS is helpful in this > context. > > >> > > >> I should have provided a little bit more guidance. I was thinking out > > >> loud in yesterday's IRC meeting. My thought was more along the lines > > >> of looking at how UIDs/naming should be done, and that specs had to > > >> solve a naming problem. However the static nature of specs is not a > > >> great fit to this problem. > > >> > > >> Actually, what is a good model would be something like /proc or > > >> Linux's sysfs. An IMFS filesystem that exports task information could > > >> be used to name memory regions. (It could eventually supplant > > >> task-based statistics reporting too.) > > >> > > >> Another idea I had though, which seems to have been lost in the > > >> shuffle, is to look at how the object names work in RTEMS and see if > > >> we can add some fixed relationships, e.g., task_name # stack. > > >> > > >> I think we should start by just treating the entire task stack as a > > >> single named object; either it is all shared, or none of it is shared. > > >> This will be easier to implement and also more widely supported by > > >> simpler MPU/MMU hardware. Later on, we can consider extending the > > >> namespace with 'offsets' /taskfs/IDLE/stack/00000A28 > > >> could be a location at byte A28 offset from the start of the stack of > > >> the IDLE task. > > >> > > > > > > I have a few questions - > > > > > > > Users would get the stack address of the stack they want to share > through pthread_attr_getstack(). Now, when they get the address they want > to share, they would pass the appropriate name of this memory-region. What > we have to provide is a mechanism to 'convert' this address to an > appropriate name. Is this the accepted way or the other way round, i.e. the > user passes a name as per a specified convention, and that name is > 'converted' to a specific address? > > > > > We may want both to work. You definitely want to have the > > address->name working though, at the very least with the base address > > returned by pthread_attr_getstack, but you might also want to be able > > to map any address in a task's stack to the stack's "name". I'm not > > sure if that is needed yet, but keep it in mind as a possible > > extension later to use an address interval instead of a fixed base > > address. > > > > One more clarification, the "name to address" conversion should be > done within the shm+mmap implementation. shm takes a name and returns > a fd, mmap takes an fd and returns an address. > Got it. > > > > When you say "treating the entire task stack as a single named > object" does it mean that we assign a single name, say "task_stack" to the > complete stack address space? In that case, how do we deal we the presence > of multiple tasks that are allocated from the same pool of task stack? I > understand that on a simpler MPU/MMU hardware it would make sense to > specify names for each memory section (.txt- "text", .bss - "bss" etc.) but > in this case, where we are sharing only selected thread-stacks, I suppose > we will have to have a way to handle 'offsets' right from the start? > > > > > > > No, I'm thinking one name for each task's stack. If you have 10 tasks, > > you'd have 10 names. Ok, so that means we can have naming like this - For an idle thread stack address we have ->/taskfs/IDLE, for a POSIX thread address we have -> /taskfs/"thread_id". Where we maintain a table for the name and its corresponding address? > > > Each allocated task stack is logically a separate region within the > > pool. For simple MPU hardware, it may not be possible to share > > arbitrary task stacks, but in that case the implementation can just > > ignore the name and share the entire pool if that is preferred, or > > return an error. (The behavior could be configurable, maybe.) > > > > >> > > >> Gedare >
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