Kernel Development Learning Pipeline

Page Walk 🚶

Add a new system call that walks the page table and translates virtual memory addresses to physical addresses

Outcomes:

• Understand how the page table maps virtual memory addresses to physical memory addresses

• Gain experience writing C code with no C runtime and no C library

What to submit:

• Patch 1 adds syscall.patch, which patches the v6.13 kernel code to implement a riscv_pagetable_walk system call

  • This patch adds the patch file you generated from the kernel repo as a .patch file to the submissions repo

    • Note that this is the only time it is acceptable to submit a patch that contains whitespace errors as this is an artifact of committing an email code patch

  • Generate a single patch from your commit to the Linux kernel using the command git format-patch -1 within your linux repo

  • Take the .patch file outputted, and put it in your named directory (e.g. cp 0001*.patch ~/submissions/$USER/pagewalk_syscall/syscall.patch)

  • Add the email patch itself as a file named syscall.patch to the staged git files and commit this

• Patch 2 adds your new init program which invokes your new system call and prints the required results accordingly

• Don’t forget a cover letter

• Submit your patches to page_walk@spring2025-utsa.kdlp.underground.software

Procedure:

• From now on when building the kernel, use a config that has the MMU enabled

  • Create a new config using ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- make menuconfig

    • Navigate to the MMU-based Paged Memory Management Support option with the up/down arrows and press y to enable the MMU

    • This also disables the option Build a kernel that runs in machine mode

  • You must remove the -bios none flag when starting a new qemu virtual machine

    • When the kernel is no longer running in machine mode, something must take responsibility for the startup process that must occur in machine mode

    • Qemu provides OpenSBI to do this by default, and we were suppressing this behavior with -bios none

• Create a new branch in the linux repo with no changes from previous assignments

  • i.e. git checkout -b pagetable_walk_syscall v6.13

• Create a new system call, riscv_pagetable_walk

  • The definition of the system call and appropriately numbered SYSCALL_DEFINE macro should be written in a new file arch/riscv/mm/riscv_pagetable_walk.c

  • This new system call takes two arguments: a void * pointer that is the virtual memory address to locate in the page table and a unsigned long __user * which is an array of 6 unsigned longs

    • This system call will walk the page table to try and locate a given pointer’s corresponding page table entry (PTE)

      • Starting with the value of the supervisor address translation and protection (SATP) register, and continuing with each PTE encountered, the system call records the data it processes

        • Each value (the SATP and the subsequent PTEs) are copied into a local array of unsigned longs

      • When the walk terminates (either by reaching an invalid mapping or locating the terminal page table entry), the system call copies the data from the local array of unsigned longs to the provided user array

        • If that copy fails, the system call returns -EFAULT

        • Otherwise, it returns the number of unsigned longs copied to the user array

    • The RISC-V privileged ISA contains all of the reference you should need

      • Section 12.3.2 describes the virtual address translation process

        • These instructions assume you are using the Sv32 virtual memory system, read the satp register yourself to determine which virtual addressing mode your virtual machine is using and find the specification for that mode

    • You will probably need the following new kernel functions and macros

      • phys_to_virt

      • csr_read

      • CSR_SATP

  • Because of the architecture-specific nature of this pagetable walk, this system call should be avialable only on RISC-V

    • The process for adding an architecture-specific system call is different from the process you followed in the new_syscall assignment

  • Add riscv_pagetable_walk.o to obj-$(CONFIG_MMU) in arch/riscv/mm/Makefile

  • Add an asmlinkage declaration of riscv_pagetable_walk in arch/riscv/include/asm/syscall.h

  • Allocate an architecture-specific system call number

    • The kernel provides a __NR_arch_specific_syscall macro

      • See the definition and a brief explanation here

    • Define __NR_riscv_pagetable_walk in tools/arch/riscv/include/uapi/asm/unistd.h

      • See the definition of __NR_riscv_flush_icache in this file for guidance

      • Assign your system call the number __NR_arch_specific_syscall + 13

        • __NR_arch_specific_syscall + 14 and __NR_arch_specific_syscall + 15 are already taken by other RISC-V-specific sytem calls

        • There is a limit of 16 architecture specific syscalls

        • Use + 13 which is available in riscv kernel v6.13

  • Add a table entry to scripts/syscall.tbl

    • Add your entry at the apropriate location according to its number

      • make sure your set the architecture to riscv

• Create a new init program to test your implementation of riscv_pagetable_walk

  • Create a new c file to act as your init program

  • When compiling your program, use riscv64-linux-gnu-gcc -nostdlib walk-init.c -march=rv64i -mabi=lp64 -o rootfs/init

  • Even though we now have an MMU, we still don’t have a C runtime

    • Instead of writing a main function, define a function void _start(void)

      • This will be your init program’s entrypoint

  • We also don’t have a standard library, so you will have to write some inline assembly or link with a separate .s file in order to create a syscall(...) wrapper function

  • Your program should exercise your new system call on pointers to different types of data

    • Your program should explain what it is testing, then print the output that it received

    • Your program should extract the physical page number (PPN), and the dirty, accessed, global, user, execute, write, read, and valid flags

  • Call your system call at least five times where each call yields a unique combination of flags

    • You shouldn’t have a hard time finding five different flag combinations by passing pointers to different kinds of data that exist in the address space of a normal C program

      • Extra credit is available if you can find more than five unique combinations of flags

    • In your cover letter when discussing your approach to completing the assignment:

      • Document how you went about finding new combinations of flags

      • Explain why each type of the data is market by its particular flags

  • Call the reboot system call

    • Use the same args as used in previous assignments to shut down: 0xfee1dead, 0x28121969, 0x4321fedc