aos lab 8: interrupts and device drivers

Lab 8: Interrupts and Device DriversAdvanced Operating Systems

Zubair Nabi

[email protected]

March 28, 2013

Recap of Lab 5: Extraordinary events

Events that break normal processor flow, to return control to thekernel:

1 A device signals that it needs attention (e.g. Timer): Interrupt

2 A user program does something illegal (e.g. divide by zero):Exception

3 A user program asks the kernel for a service: System call

Recap of Lab 5: Handling extraordinary events

The operating system must:

1 Save the processor’s registers for future resumption

2 Set up system for execution in the kernel

3 Choose a place for the kernel to start execution

4 Retrieve information about the event and call correspondinginterrupt handler

5 All the while, maintain isolation between user processes and thekernel

Recap of Lab 5: Handling extraordinary events (2)

• Need hardware support

• On the x86, system calls generate an interrupt via the intinstruction

• The same mechanism for handling interrupts is used for handlingsystem calls and exceptions

• Traps are caused by the current running process• Interrupts are caused by devices

• Can happen concurrently

Recap of Lab 5: trap

• Gets passed struct trapframe *tf

• Checks tf->trapno to decide if it was called for a system call(T_SYSCALL) or a hardware interrupt or an exception

• In case of:1 System call, it invokes syscall2 Hardware interrupt, it calls the hardware interrupt controller3 Exception, it prints the details and kills the user process

Interrupts

• Examples of interrupts: Pressing a key on the keyboard, storingdata on disk etc.

• Hardware on the motherboard signals the CPU whenever adevice needs attention

• Devices need to be programmed to generate interrupts

• Interrupts must be assigned to a CPU for handling

Interrupts

Handing interrupts on multi-processors

Two parts of handling interrupts on multi-processors

1 A method to route interrupts to processors (I/O APIC – AdvancedProgrammable Interrupt Controller, ioapic.c)

2 A per CPU interrupt controller to handle interrupts (Local APIC,lapic.c)

The OS needs to program both the IOAPIC and the LAPIC

IOAPIC

• Interrupts are generated by the APIC labelled from IRQ0 toIRQ16 or IRQ24

• A device enables particular interrupts within the IOAPIC table andspecifies which processor they should be routed to

• For instance, the keyboard interrupt is routed to CPU0 while diskinterrupts are routed to CPU(n−1), where n is the number ofCPUs

• Every CPU decides whether it wants to receive interrupts. Forinstance, scheduler() disables all interrupts

IOAPIC

• Each CPU has its own LAPIC

• xv6 programs the LAPIC using lapicinit()

• Each CPU has its own LAPIC

• xv6 programs the LAPIC using lapicinit()

Example: Timer interrupt

• The kernel uses timer interrupts to get a notion of time and tomake scheduling decisions

• A value of 100 timer ticks is used by default which is high enoughto allow interactivity and low enough not to lead to a live lock

• This value is set in lapicinit() and IRQ_TIMER is mappedto IRQ0

• IRQ_TIMER corresponds to interrupt number 32

• In trap(), for interrupt number 32, the timer tick is incrementedand processes sleeping on it are woken up

Device Driver

• A piece of code in the OS that manages a particular device• Provides interrupt handlers for a device• Causes a device to perform operations• Causes a device to generate interrupts

• Non-trivial to write as a driver executes concurrently with thedevice that it manages

• Driver also needs to understand the hardware interface of thedevice

Device Driver

Example: Disk driver

• In charge of copying data back and forth from the disk• Data is represented on the disk as a sequence of sectors, each

containing 512-byte blocks• Sector 0 represents the first 512 bytes, sector 1 the next 512, and

• The OS has a corresponding structure (struct buf) thatmaps to one sector

• The data within the OS sector structure is often out of sync withthe disk

• xv6 implements an IDE driver

Code: struct buf

struct buf {int flags;uint dev;uint sector;struct buf *prev;struct buf *next;struct buf *qnext;uchar data[512];

};#define B_BUSY 0x1#define B_VALID 0x2#define B_DIRTY 0x4

Example: Disk driver initialization

• ideinit() is invoked by main(), which initializes the diskdriver

1 ideinit() sets up CPU(n−1) to handle disk interrupts2 It then makes a call to idewait() to wait for disk 0 to initialize

• It assumes that at least disk 0 is present because the boot loaderand the kernel were obviously loaded from it

3 idewait() polls the status bits of the disk hardware, untilIDE_BSY is clear and IDE_DRDY is set

4 It then checks whether another disk is present

Buffer cache

• The buffer cache is a linked list of struct buf in memory

• It holds cached copies of disk blocks• Uses iderw(struct buf *b) to read/write a buffer from/to

the disk• If B_DIRTY is set, it writes b to disk• If B_VALID is not set, it reads b from disk

Buffer cache

• Disk accesses take milliseconds of time therefore polling is not anoption

• iderw keeps the list of pending disk requests in a queue(idequeue) and uses interrupts to signal whenever a requesthas finished

• It adds b to the end of idequeue• If b is the only buffer in idequeue, it instantly dispatches it to the

disk hardware by calling idestart() which either issues aread or a write

• The current process is then put to sleep waiting for the operationon b to be completed

• Once the hardware has completed the operation, it will generatean interrupt (IRQ_IDE) which will be handled by trap byinvoking ideintr()

Code: iderw

void iderw(struct buf *b){

struct buf **pp;acquire(&idelock);b->qnext = 0;for(pp=&idequeue; *pp; pp=&(*pp)->qnext)

*pp = b;if(idequeue == b)

idestart(b);while((b->flags & (B_VALID|B_DIRTY)) != B_VALID){

sleep(b, &idelock);}release(&idelock);

ideintr

1 Checks the first buffer in idequeue to find the type of theoperation (read or write)

2 If the operation was a read, it reads the data into the buffer viainsl()

3 It then sets B_VALID and clears B_DIRTY and wakes up anyprocesses sleeping on b

4 Finally, it dispatches the next buffer from idequeue

ideintr

Modern disk controllers in a real OS

• Most disk controllers typically accept multiple outstanding diskrequests at a time

• They even reorder them to make efficient use of the disk arm• Conceptually other hardware is very similar to disks

• Network buffers hold packets• Audio device buffers hold sound samples• Graphics card buffers hold video data

• High-bandwidth I/O devices, such as disks, graphics cards,network cards, etc. often use direct memory access (DMA)instead of explicit I/O (insl and outsl)

• Via DMA, controllers get direct access to physical memory

• The device driver passes the physical address of the buffer’s datafield to the device, which copies it from/to the main memory

• Once it is done, it generates an interrupt

• As a result of DMA, the CPU is not involved, hence making theentire process more efficient

Dynamic interrupt routing

• In case of xv6 all disk interrupts are routed to CPU(n−1)• Real OSes use sophisticated algorithms to dynamically route

interrupts to processors• The goal is to achieve optimum load balancing while maintaining

good locality

Reading(s)

• Chapter 3, “Traps, interrupts, and drivers”, section “Code:Interrupts" onwards from “xv6: a simple, Unix-like teachingoperating system”

aos lab 8: interrupts and device drivers

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