xv6 reading homework

Throughout this semester, we expect that you will read the entire implementation of the xv6 kernel, accompanied by the xv6 book, which explains the rationale and design underlying the kernel's implementation. (The source code is also available as a Lions-style PDF)

For this lecture, read the following files in the xv6 kernel implementation:

• kernel/riscv.h
• kernel/trampoline.S
• kernel/trap.c

Build the xv6 kernel:

$ make kernel/kernel 

Open the file kernel/kernel.asm, and look for the line _trampoline, which is where the file trampoline.S is loaded. You will find at address 80006092 the instruction that loads the kernel page table (the exact addresses may be different ):

0000000080006000 <_trampoline>:
    80006000:   14051073                csrw    sscratch,a0
        ...
	...
    80006092:   18031073                csrw    satp,t1
    80006096:   12000073                sfence.vma
        ...

After the CPU executes csrw satp, t1, the CPU will translate 0x80006096, which holds the next instruction, using the kernel page table (t1 holds the address of the kernel page table; see trampoline.S). How come that this address 0x80006096 in both the user and the kernel page table translates to a physical address that holds sfence.vma?

You may find chapter 4 of the book useful in understanding how transition between user space and kernel space on a RISC-V CPU.

Submit your answer in an ASCII text file named homework.txt to the corresponding "Lecture N" assignment on Gradescope.

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