Why??

I am currently going through the book Operating Systems: Three Easy Pieces, and the next step after completing the first piece, Virtualisation, is to go through the xv6 kernel. The xv6 kernel had two implementations, the x86 one and RISC-V one. As the x86 one is now unmaintained and on recommendation of a mentor, I am using xv6-riscv! So without further ado, let’s get started.

The xv6 Kernel

Based on Unix kernel
Created by MIT for educational purposes
Implemented for RISC-V
Multicore
~6000 lines of code (C and Assembly)

Features

Processes
Virtual Address Spaces
- Page Tables
Files and Directories
Pipes
Multitasking
- Time-Slicing
21 system calls

User Programs

sh
cat
echo
grep
kill
ln
ls
mkdir
rm
wc

What is missing?

All the complexity of a “real” operating system is missing. For example, UserID, file protections, IPC, “mount”able filesystem, etc.

SMP: Shared Memory Multiprocesser

CPU = core = HART

Main memory (RAM) is shared and is of size 128 Mbytes

Supported Devices

UART (Serial Comm. Tx <=> Rx)
Disk
Timer Interrupts
PLIC: Platform Level Interrupt Controller
CLINT: Core Local Interrupt Controller

Memory Management

Page Size = 4096 bytes (#define PGSIZE)
Single Free List
No variable sized allocation
No “malloc”
Page Tables-
- Three Levels
- One table per process + One table for Kernel
- Pages marked - R/W/X/U/V

Scheduler

Round Robin
Size of TimeSlice is fixed (1,000,000 cycles)
All cores share one “Ready Queue”
Next TimeSlice may be on a different core

Boot Sequence

QEMU
- Loads kernel code at a fixed address (0x8000-0000)
- Starts all cores running
No bootloader/boot-block/BIOS

Locking

Spin Locks
sleep() & wakeup()

”Param.h”

Fixed Limits (like no. of processes, no. of open files, etc)
Several Arrays (“kill(pid)” => Linear search of Processes array)

User Address Space

Arguments (argc, argv) will be placed on the stack before the program begins execution.

RISC-V Virtual Addresses

Multiple Schemes are available
- Sv32 -> Used for 2-level Page Tables
- Sv39 -> For 3-levels
- Sv48 -> For 4-levels!
xv6 uses Sv39

VA Size-

  39 bits
  2^39 = 512 GB
       = 0x80-0000-0000

But xv6 uses only 38 bits for its VAs

  38 bits
  2^38 = 256 GB
           = 0x40-0000-0000

Therefore the allowed range is - 0…0x3F-FFFF-FFFF

Startup

We go from

     entry.S         ->          start.c             ->            main.c
   (Setup Stack)              (Machine Mode)                  (Supervisor Mode)

# xv6 - Introduction