Xv6 Operating System Organization

(Updated: )

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Operating System Organization

参考 xv6-riscv-book Chapter 2 Operating system organization

Three requirements for OS:

  • multiplexing
  • isolation
  • interaction

Abstracting physical resources

Transparency: simplifies interaction

  • Abstract the resources into services
  • Applications don’t have to be aware of time sharing
  • Allows the OS to decide the usage of memory

User mode, supervisor mode, and system calls

A kernel and two user processes

Kernel organization

Organization Description Upside Downside
monolithic kernel the entire OS resides in the kernel with full privilege convenient,
easier for different parts of the OS to cooperate		 complex interfaces,
easy to make a mistake,
a mistake is fatal
microkernel minimize the code that runs in supervisor mode,
execute the bulk of the OS in user mode		 reduce the risk of mistakes in the kernel

A microkernel with a file-system server

(NOTE> OS services running as processes are called servers)

Xv6 organization

Xv6 kernel source files:

File Description
bio.c Disk block cache for the file system.
console.c Connect to the user keyboard and screen.
entry.S Very first boot instructions.
exec.c exec() system call.
file.c File descriptor support.
fs.c File system.
kalloc.c Physical page allocator.
kernelvec.S Handle traps from kernel, and timer interrupts.
log.c File system logging and crash recovery.
main.c Control initialization of other modules during boot.
pipe.c Pipes.
plic.c RISC-V interrupt controller.
printf.c Formatted output to the console.
proc.c Processes and scheduling.
sleeplock.c Locks that yield the CPU.
spinlock.c Locks that don’t yield the CPU.
start.c Early machine-mode boot code.
string.c C string and byte-array library.
swtch.S Thread switching.
syscall.c Dispatch system calls to handling function.
sysfile.c File-related system calls.
sysproc.c Process-related system calls.
trampoline.S Assembly code to switch between user and kernel.
trap.c C code to handle and return from traps and interrupts.
uart.c Serial-port console device driver.
virtio_disk.c Disk device driver.
vm.c Manage page tables and address spaces.

(From: pbpaste | awk '{ printf("| %s |", $1); for (i=2; i<=NF; i++) printf(" %s", $i); printf(" |\n"); }' | pbcopy)

The inter-module interfaces are defined in kernel/defs.h.

Process overview

Process

The unit of isolation: a process: an illusion to a program that it has its own private machine (private memory, CPU, file descriptors, etc.). Process is defined as struct proc (kernel/proc.h:86).

  • p->state: whether the process is allocated, ready to run, running, waiting for I/O, or exiting:
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enum procstate { UNUSED, USED, SLEEPING, RUNNABLE, RUNNING, ZOMBIE };
  • p->pagetable: holds the process’s page table.

Thread

Thread (of execution): executes the process’s instructions.

A thread can be suspended and later resumed.

Threads can “block” in the kernel to wait for I/O, and resume where it left off when the I/O has finished.

Virtual address

Virtual address: Isolation of memory: virtual address -- page tables --> physical address: 

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MAXVA
↑    trampoline
|    trapframe
|    Heap
|    Stack (user stack)
|    Global Variables (text and data)
|    Instructions
0
  • VA is starting at zero
  • MAXVA (the maximum virtual address) defined in kernel/riscv.h:348: Xv6 uses 38 bits to look up virtual addresses in page tables: $\textrm{MAXVA}=2^{38}-1=\textrm{0x3fffffffff}$
  • Each process has two stacks: user stack & kernel stack (p->kstack, for a system call or interrupt, separate and protected from user code).

System call

ecall: a RISC-V instruction to make a system call:

  1. raises hardware privilege level
  2. change PC to a kernel-defined entry point, switches to a kernel stack
  3. executes the kernel instructions
  4. (system call completes) switches back to the user stack
  5. returns to user space by calling the sret instruction (lowers the hardware privilege level)
  6. resumes executing user instructions just after the system call instruction

Starting xv6 and the first process

  1. RISC-V computer powers on, self initializes
  2. runs a boot loader (stored in ROM): loads the xv6 kernel into memory at physical address 0x80000000 (range 0x0:0x80000000 contains I/O devices)
  3. (in machine mode) executes xv6 starting at _entry (kernel/entry.S:6), sets up a stack (stack0) for C code (sp = stack0 + (hartid * 4096))
  4. _entry calls into C code: function start (kernel/start.c:11)
  5. start performs configuration(page-table, interrupts…)
  6. switches to supervisor mode, PC change to main (kernel/main.c:11)
  7. main initializes several devices and subsystems
  8. creates the first process by calling userinit (kernel/proc.c:212)
  9. run initcode.S (user/initcode.S:1), do exec("/init")
  10. init (user/init.c:15) creates a console device file, opens it as file descriptors 0, 1, and 2
  11. starts a shell on the console
  12. The system is up.

EOF

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// By CDFMLR 2021-02-27
printf("See you.\n");

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