Week 10.a
CS6640
11/07 2023
https://naizhengtan.github.io/23fall/

1. RV32 page table intro
2. (manually) walking page table
3. OS implementation
   - trapping to kernel
   - message between
   - free all pages
---

Admin:
- final oral exam
- hacking day on Thursday
- midterm next week

1. Intro

  [show overview fig]

  * a toy example:
    VA: 0x80001fec

  * split it into L1/L2 index and offset:
   0x80001fec =>
     - l1: 1000 0000 00 => 0x200 => 2*16*16=512
     - l2: 0x1 => 1
     - offset: 0xfec

  detail info:
  (1) satp
    [handout]

  (2) PTE
    [handout]

  Q: what if a PTE has W but not R?
    [undefined behavior]

  (3) VA
    [handout]

2. walking page table

   * a real example
     take a look at helloworld.c

   int main(int args, char **argv) {
       m_uint32 data = 0xdeadbeef;
       printf("%p\n", &data);

       asm("ecall");
   }

   * run it; it returns

   0x80001fec
   // error

   Q: what do you think
   gdb> p/x *0x80001fec
   [A: 0]

   Q: Why? We're in M-mode. Only see physical memory.

  * simulate CPU: manual page walk

    Goal: see which physical address holds data "0xdeadbeef".
    Method: we will use gdb to simulate page walk.
    Steps:
      (1) split the VA to l1/l2 indexes and offest
      (2) get the root of page table
      (3) calc the L1 page
      (4) calc the L2 page
      (5) calc the physical address

  * now, let's do it:

 (1) split the va to l1/l2 indexes and offest

    0x80001fec =>
      l1: 0x200 (512)
      l2: 0x1
      offset: 0xfec

 (2) get the root of page table
   gdb> p/x $satp
    // 0x80080048

 (3) find the L1 page:

   Q: how to interpret 0x80080048?
   [the most significant bit is MODE]

   Q: what is the L1 page?

     gdb> p/x 0x80048 << 12
     // 0x80048000

   Q: is this physial or virtual?
      [physical]

   Q: what is the L1 PTE address?

     gdb> p/x (0x80048 << 12) + 0x200*4
     // 0x200 is the L1 index in VA

   Q: what is the L1 PTE content?

     gdb> p/x *((0x80048 << 12) + 0x200*4)
     // get 0x20014401 (L1 PTE)

   Q: what does "0x1" in the end mean?
     [it is the valid bit]

 (4) find the L2 page:

   Q: what's the L2 page?

     gdb> p/x (0x2001401 & ~(0x3ff)) >> 10 << 12
     // 0x80051000

   Q: what's the L2 PTE address?

     gdb> p/x 0x80051000 + 0x1*4
     // 0x1 is the L2 index in VA

   Q: what's the L2 PTE content?

     gdb> p/x *(0x80051000 + 0x1*4)
     // 0x200158df

 (5) find the physical address

   Q: what is "0xdf" in 0x200158df?
     [check out PTE fig]

   Q: what is the data page?

     gdb> p/x (0x200158df & ~(0x3ff)) << 2
     // 0x80056000

   Q: what is the PA?

     gdb> p/x ((0x200158df & ~(0x3ff)) << 2) + 0xfec
     // 0x80056fec
     // you will see 0xdeadbeef (the data) when inferencing the address


3. OS implementation

  A. trap to kernel

   setup:
    gdb> add-symbol-file build/release/helloworld.elf
    gdb> display proc_curr_idx
    gdb> display/x $satp
    gdb> display/x $sp
    gdb> display/x $pc

   Q: what do you think of "satp", "sp", "pc" when trapping?
    - "satp" and "sp" will not change
    - "pc" points to the "mtvec"
    - the virtual address space will be shut down (running in M-mode)

  B. message passing

    Q: how to copy message across processes?

  C. free page table

    Q: how to free all pages of a process?