| .. _mpu_userspace: |
| |
| MPU Backed Userspace |
| #################### |
| |
| The MPU backed userspace implementation requires the creation of a secondary |
| set of stacks. These stacks exist in a 1:1 relationship with each thread stack |
| defined in the system. The privileged stacks are created as a part of the |
| build process. |
| |
| A post-build script :ref:`gen_priv_stacks.py` scans the generated |
| ELF file and finds all of the thread stack objects. A set of privileged |
| stacks, a lookup table, and a set of helper functions are created and added |
| to the image. |
| |
| During the process of dropping a thread to user mode, the privileged stack |
| information is filled in and later used by the swap and system call |
| infrastructure to configure the MPU regions properly for the thread stack and |
| guard (if applicable). |
| |
| During system calls, the user mode thread's access to the system call and the |
| passed-in parameters are all validated. The user mode thread is then elevated |
| to privileged mode, the stack is switched to use the privileged stack, and the |
| call is made to the specified kernel API. On return from the kernel API, the |
| thread is set back to user mode and the stack is restored to the user stack. |
