Xv6 Interrupts and device drivers

Learning: xv6-riscv-book Chapter 5 Interrupts and device drivers

Driver:

• code in an OS
• manages a particular device
  • config hardware
  • tells operations
  • handles resulting interrupts
  • interacts with processes

Devices -> driver -> interrupts (one type of trap) -> OS

xv6 handles driver’s interrupt in devintr (kernel/trap.c:117)

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Common driver: two contexts:

context	runs in	called	perform
top half	a process’s kernel thread	called via syscalls (e.g. read/write)	- ask hardware to start an operation - waits for operation to complete
bottom half	excutes at interrupt time	device completes the operation => raise an interrupt	as interrupt handler: - what operation has completed? - wakes up a waitting proc - tells hw to do next operation

Console input

console.c: console driver: a simple illustration of driver structure.

• accepts typed chars via UART hardware
• accumulates a line
• processing sepcial input chars (e.g. ^H for backspace, ^D for End-of-Transmission)
• user processes use read to fetch inputed lines.

QEMU:

• keystrokes -> QEMU’s simulated UART hardware

UART:

• (QEMU hardware): emulated 16550 chip connected to keyboard and display.

• (appears to software): a set of memory-mapped control register. (kernel/uart.c)

  • LSR: wheter input chars are waiting to be read
  • RHR: chars to read (FIFO, if empty -> LSR.ready = 0)
  • …

• initialize: consoleinit (kernel/console.c:182)

  • generate receive interrupts when the UART receive bytes from input
  • generate transmit complete interrupts when finished sending a byte of output

processes (e.g. shell):

• [process] read from console (via file descriptor) ->
• [kernel] consoleread ->
• [consoleread] waits input (sleep to wait user typing) to arrive cons.buf, copies input to user space until \n ->
• [kernel/consoleread] returns to user process

User types a character:

• UART hardware ask RISC-V to raise an interrupt
• activates xv6’s trap handler
• handler calls devintr
• devintr looks scause: discover interrupt is from a device
• devintr ask PLIC hardware to tell which device interrupted
• if it was UART: calls uartintr
• uartintr reads input chars from UART hw
• hands chars to consoleintr
• consoleintr: accumulate input chars in cons.buf until a whole line arrives
• treats special chars (e.g. backspace)
• when a newline arrives: consoleintr wakes up a waiting consoleread

Console output

• write to a fie descriptor connected to console -> uartputc
• uartputc puts each char into an output buffer (uart_tx_buf), calls uartstart
• if buffer full -> uartputc waits
• uartstart: if device ready -> transmitting
• UART finishes sending a byte -> generates an interrupt
• interrupt -> uartintr -> uartstart

Concurrency in drivers

I/O concurrency:

• the decoupling of device activity from process activity via buffering and interrupts.
  • console driver can process input even when no process is waiting to read it
  • processes can send output without having to wait for the device

Concurrency dangers:

• two processes on different CPUs call consoleread at the same time.
  • solve: acquire a lock
• one process waits for input, interrupt of the input arrive when a different process is running.
  • interrupt handlers copy input data to a buffer
  • wake up top-half code to do the rest (consoleintr -> consoleread)

Timer interrupts

Clock hardware:

• attached to each RISC-V CPU
• xv6 programs this clock hw to interrupt each CPU periodically

Timer interrupt:

clock hardware -> timer interrupt -(trap)-> usertrap / kerneltrap -> yield -> switch processes

RISC-V timer interrupts: can only be taken in MACHINE MODE, not supervisor mode:

• xv6 handles timer interrupts separately from the trap mechanism.
  • before main -> start.c -> timerinit: sets up to receive timer interrupts in machine mode
    • program the CLINT hardware (core-local interruptor) to generate an interrupt after a delay
    • set up a scratch area (like trapframe): help the time interrupt handler save registers and address of CLINT
    • enables timer interrupt: mtvec <- timervec
  • timer interrupt (timervec: machine mode) -> handler: raise a software interrupt and return -> delivers softwate interrupt (devintr) with ordinary trap mechanism

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EOF

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# By CDFMLR 2021-03-30
echo "See you."

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