Hi all,
Sorry to bother, I have some basic questions about seL4 memory management. Can someone help me?
1. where is the kernel’s address space?
I know there are basically three methods:
a) separate virtual address space, e.g. through changing page tables on entry into privileged mode to access
b) physical space, e.g. through disable automatic hardware translation of virtual addresses on entry into privilege mode
c) privilege region in each process’s virtual address space, e.g. through page table to map kernel address into use-mode process.
My question is which of this is being used by seL4 microkernel?
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2. Objects like CSpace, VSpace, CNode, TCB, Endpoint, etc. are called kernel objects, but it seems all dynamically allocated in thread running/creating time. Are those objects (e.g. Figure 3.3 in seL4 manual) in kernel space or in
user space?
The memory for these objects is in kernel space (the user can't access them), but the user is responsible of allocating the memory for such objects (using retype untype), and give it to the kernel. Any operation on such objects requires caps.
3. I read that after the kernel boot up, it passes all resources, untyped memory to the first thread. Are those untyped memory physical memory or virtual memory in a single page?
The reason I ask is I’m learning the code of the UNSW AOS project (two of the assignment projects are implementing frame and papers). I’m confused that before the implementation of frames and pages what kind of memory is passed to the the first
thread? I saw int AOS project, it uses ut_table_init(), ut_steal_mem(), I wonder what kind of memory it operates on?
The root thread is granted system resources (including untyped memory). Information about untyped memory is held by seL4_BootInfo and contains "physical" start and end addresses for each untyped region. No virtual addresses are involved. For example, if you
want to read/write to some (untyped) memory, you have to retype_untype it into a frame, and then map it to your address space.
Thanks
-Dan
Cheers,
Hesham