Tasks

Enviroment preparation

Read the Install toolchain section to set up your local development environment and get familiar with the course project.

Design Doc

Download the design document template of lab 0. Read through questions in the document to get a feel for the lab before you start, and answer them as you go.

Booting Tacos

If you have set up the Tacos development environment as described in the Install toolchain section, you should see Tacos booting in the QEMU emulator, and print SBI messages, "Hello, world!" and "Goodbye, world!". Take screenshots of the successful booting of Tacos in QEMU.

Debugging

While you are working on the projects, you will frequently use the GNU Debugger (GDB) to help you find bugs in your code. Make sure you read the Debugging section first. In addition, if you are unfamiliar with RISC-V assembly, the RISC-V Instruction Set Manual is an excellent reference. Some online resources, such as Five Embeddev, are also available.

In this section, you will use GDB to trace the QEMU emulation to understand how Tacos boots. Follow the steps below and answer the questions in your design document:

  1. Figure out the right command to start Tacos, freeze it, and use gdb-multiarch to connect to the guest kernel.

Question

  • What is the first instruction that gets executed?
  • At which physical address is this instruction located?

Note: Zero Stage Bootloader (ZSBL)

If you use commands in Debugging to start Tacos & freeze Tacos's execution, QEMU will wait at the very first instruction, which will be executed by the machine later. The first few executed instructions are called (at least in the RISC-V world) the Zero Stage Bootloader (ZSBL). In real machines, the ZSBL is most likely located in a ROM firmware on the board. The ZSBL reads some machine information, initializes a few registers based on them, and then passes control to SBI using jmp instructions.

  1. QEMU's ZSBL only contains a few (less than 20) instructions, and ends with a jmp instruction. Use gdb to trace the ZSBL's execution.

Question

  • Which address will the ZSBL jump to?
  1. Before entering the main function in main.rs, the SBI would have already generated some output. Use breakpoints to stop the execution at the beginning of the main function. Read the output and answer the following questions:

Question

  • What are the values of the arguments hart_id and dtb?
  • What are the values of Domain0 Next Address, Domain0 Next Arg1, Domain0 Next Mode and Boot HART ID in OpenSBI's output?
  • What's the relationship between the four output values and the two arguments?
  1. The functionality of SBI is basically the same as BIOS. e.g. When Tacos kernel wants to output some string to the console, it simply invokes interfaces provided by SBI. Tacos uses OpenSBI, an open source RISC-V SBI implementation. Trace the execution of console_putchar in sbi.rs, and answer the following questions:

Question

  • Inside console_putchar, Tacos uses the ecall instruction to transfer control to SBI. What are the values of register a6 and register a7 when executing that ecall?

Kernel Monitor

Finally, you will get to make a small enhancement to Tacos and write some code!

Note: Features in Tacos

  • When compiled with different features, Tacos can do more than just printing "Hello, world!".
  • For example, if test feature is enabled, Tacos will check for the supplied command line arguments stored in the kernel image. Typically you will pass some tests for the kernel to run.
  • There is another feature named shell. To build and run the kernel with shell feature, you could use cargo run -F shell.

Currently, the kernel simply terminates after booting. This is a little boring. Your task is to add a tiny kernel shell to Tacos so that when compiled with the shell feature, Tacos will run this shell interactively.

Note: Kernel shell

  • You will implement a kernel-level shell. In later projects, you will be enhancing the user program and file system parts of Tacos, at which point you will get to run the regular shell.
  • You only need to make this monitor very simple. The requirements are described below.

Requirements:

  • Enhance main.rs to implement a tiny kernel monitor in Tacos.
  • It starts with a prompt PKUOS> and waits for user input.
  • When a newline is entered, it parses the input and checks if it is whoami. If it is whoami, print your student id. Afterward, the monitor will print the command prompt PKUOS> again in the next line and repeat.
  • If the user input is exit, the monitor will quit to allow the kernel to finish. For other inputs, print invalid command.

Hint

  • The code place for you to add this feature is in main.rs: // TODO: Lab 0
  • You may find some interfaces in sbi.rs to be very useful for this lab.