One key feature of Tock is the ability to load and run multiple applications simultaneously. In a modern computer, the OS uses the Memory Management Unit (MMU) to provide a virtual address space for each process. The code for each process is written assuming that it is the only code in the world and that it can place memory at any address it pleases. The MMU handles translating these virtual addresses into a physical address in real memory, which is shared between all processes. Unfortunately, in the world of embedded systems MMUs are not available. Processors like the ARM Cortex-M series omit them since they are power and area-hungry.
A common approach to handle this issue is to assign addresses to applications at compile-time. For example, Application A can be placed at address 0x21000 and Application B can be placed at address 0x22000. This runs into problems when Application A grows in size in the future. Moreover, with Tock we want to be able to handle dynamically adding, updating, and removing applications at run time. Assigning each an address in advance simply isn’t possible.
In Tock, we use position independent code (PIC) 1 to enable loading multiple applications. In PIC, all branches and jumps are PC-relative rather than absolute, allowing code to be placed at any address. All references to the data section are indirected through the Global Offset Table (GOT) 2. Rather than access data at an absolute address, first the address of the data is loaded from a hard-coded offset into the GOT, and then the data is accessed at that address. This allows the OS to simply relocate all addresses in the GOT at load time based on the actual location of SRAM rather than fixing various instructions throughout the code. The address of the GOT itself is stored in a PIC base register which is set by the OS before switching to application code and is different for each application. The ARM instruction set is optimized for PIC operation, allowing most code to execute with little to no cost in number of instructions.
While PIC handles the majority of addressing issues, it does not fix everything. Data members which are themselves pointers are assigned a value by the compiler rather than indirecting through the GOT. For example, take the statement:
const char* str = "Hello";
The address of
"Hello" is stored in the data section (as the value of
str) and is not relocated like elements of the GOT. In order to solve
this issue, we collect the addresses of necessary relocations (such as
the address of
"Hello") from the ELF and append them to the binary.
Tock can then fix each address at load time based on the actual location
of the application’s text and data segments. Since the application is
already compiled as PIC, remaining fixes will only exist in the data
segment, making the process of applying relocations simple.
Combining these two solutions together, we reach the Tock application format. Each application binary is compiled as position independent code 3, has a relocation section appended to it 4, and begins with a header structure containing the size and location of the text, data, GOT, and relocation segments as well as an entry point for the app 5. The app is able to be loaded into any flash and SRAM addresses with no control-flow costs, the recurring cost of additional load instructions when indirecting data accesses through the GOT, and the one-time cost of several simple data address relocations at application load time.
When receiving an application binary, Tock assigns space in flash and SRAM for it, loads the data segment into SRAM, fixes up addresses stored in the GOT 6, walks the relocation section fixing up additional items in the data section 7, sets the process PC to the entry point of the application, and enqueues the newly created processes to be run. Applications can be received through many methods including wireless uploads over protocols such as IEEE 802.15.4 and Bluetooth Low Energy, but the currently implemented system for Tock receives application binaries over a UART serial connection.
The main issue with this loading strategy is that it is not currently possible for applications compiled with LLVM. LLVM does not support a dynamic PIC addressing scheme like GCC’s base-register. In many cases, encoding the location of the GOT in the text segment works just fine, since it can always be when the code is loaded into RAM. However, Tock executes applications directly from flash where it is not practical to rewrite pointers dynamically.
A patch 8 to add a base-register PIC strategy was sent to LLVM a while back but it was never merged. Ironically, this means that although the Tock kernel is written in Rust, for now it isn’t possible to write applications in Rust.