2018-01-31 06:58:22 +08:00
|
|
|
.. _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 ``gen_priv_stacks.py`` scans the generated
|
2018-02-23 02:17:53 +08:00
|
|
|
ELF file and finds all of the thread stack objects. A set of privileged
|
2018-01-31 06:58:22 +08:00
|
|
|
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.
|
|
|
|
|
|
