Saturday, August 24, 2013

libmm design on RTEMS for ARM architecture

First, if you do not know what libmm is about you should review that introduction post.
In this topic, I will give a brief overall picture of how libmm is designed and some implementation specific details for ARM.

Changes : 

Some changes were made to the API mentioned in the previous post. The major change is that both _Memory_management_Install_entry and _Memory_management_Uninstall_entry interfaces were merged and unified into a new interface called _Memory_management_Set_attributes. The new function takes three arguments :
base : Which is the starting address of the memory region to apply new attributes on.
size : The size of the memory address in bytes.
attributes : Abstract target-independent high-level attributes to be applied for a memory region. This argument describes common memory attributes for memory protection and caches.

The reason of the this re-factoring is that we got rid of control structs at high-level and low-level layers, So there is no "entries" to be installed or uninstalled anymore. This interface is similar to POSIX memory protection interface: mmprotect.

Design :

There are three major layers in RTEMS for libmm: score layer, cpu layer, low-level (implementation) layer.
score layer is the front-end API which the developer should deal with. The functions of the score layer were described in the introduction post and changes are mentioned here too. It also acquires locks to manage libmm implemented for SMP environment (like realview). cpu layer is a shared header file that is considered a link or a bridge between high-level score layer and implementation. All BSPs and targets that implement libmm should include this file. implementation layer is the target-dependent code that interfaces directly with hardware memory management system for a specific target. Figure1 shows the three layers and relations between each of them. Arrows indicate that one layer (on the "from" side of the arrow) depends another layer (on the "to" side of it).

Figre1: Relations between libmm layers.

ARM Implementation :


Thanks to the good ARM architecture design and backward compatibility, RTEMS can share code for different architectures that deal with memory management part. This way, we can avoid low-level/libmm code duplication for different ARM BSP on RTEMS. libmm depends mostly at coprocessor15 for ARM targets that have MMU to configure memory management systems.

As an example of code shareability, is that RTEMS can use the same peace of code to initialize MMU and Caches at BSP startup as well as dynamically changing memory attributes at page tables. At the time of writing this post, RaspberryPi RTEMS BSP which has ARM1176JZF-S (ARMv6) processor share the same code with RealView BSP which has a Cortex™-A9 (ARMv7) processor. 

Performance and page sizes

For RTEMS, as being RTOS, the most important aspect to consider when making an implementation oriented decision is the performance and the speed. ARM MMU has two levels of page tables, each of which can describe memory attributes for different region sizes. The first level page-table can describe 16 MB super-sections or 1 MB sections. The second level page-table can describe 64 KB large pages or 4 KB pages. To keep TLB misses as few as possible beside avoiding the overhead of memory references and page table walks, we chose 1 MB sections as a basic unit for memory regions. libmm aligns up any region to 1 MB boundary.

Supported attributes

 Currently the supported attributes at high-level libmm layer are read-only, write-enable and no-access. These attributes can be easily extended to support Cache attributes too.

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