W4118 Operating Systems Interrupt and System Call in Linux

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W4118 Operating Systems Interrupt and System
Call in Linux

• Instructor Junfeng Yang

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Logistics

• TAs
• Supreeth Subramanya
• Office Hours M 3-5pm
• Address CEPSR 7LW1
• Yunling Wang
• Office Hours W 1-3pm
• Address TA room (Mudd 122A)
• Heming Cui
• Office Hours F 4-6PM
• Address TA room (Mudd 122A)

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Logistics (cont.)

• Textbooks
• Bookstore is working on the order
• Weve included the problem statements in homework
  1 page

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Homework 1 clarifications

• Your shell should wait for command to finish
• While command running, dont prompt or accept new
  command
• NOTE wait for the entire pipeline to finish
• When do IO redirection and pipe conflict?
• Tie two things to one file descriptor
• Bad ls gt 1.txt grep FOO
• bad ls sort lt file.txt
• Different shells handle conflicts differently
• tcsh emits error. Ambiguous output redirect.
• bash is silent.
• Your shell should emit an error.
• Any questions?

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Last lecture

• OS event driven
• Events from device interrupt
• Computer organization CPU, device, memory, bus
• CPUs fetch-execute cycle
• How to start this cycle boot process
• Devices need CPUs immediate attention. How?
  interrupt
• How it works
• PIC translates IRQs to interrupt
• CPU looks up handler in Interrupt Descriptor
  Table
• Traps (or Exceptions) raised inside CPU

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Last lecture (cont.)

• Events from application system call
• Often implemented via trap, e.g. int 0x80 in
  Linux
• The need for protection
• Dual-mode operation user mode and kernel mode
• Privileged instructions can only execute in
  kernel mode
• Apps transit into kernel via system calls, so
  kernel can validate the calls and perform
  privileged instructions for them
• OS structure
• Simple
• Layered

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Today

• OS structure (cont.)
• Monolithic kernel v.s. Microkernel
• Virtual machines
• Intro to Linux
• Interrupts in Linux
• System calls in Linux

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Monolithic kernel

• All OS components run in kernel mode
• Why good?
• Can be efficient. Cross-component access cheap
• Why bad?
• No boundaries ? Big, complex kernel ? hard to
  change
• Hard to do new stuff in OS ? OS researchers
  unhappy
• No flexibility for apps. Hard to customize for
  speed (web server)
• Trusted computing base (TCB) large, one error ?
  entire kernel crash, or be compromised

APP
User mode
FS
Net
Mem
Kernel mode
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Microkernel

• Moves as much from the kernel into user space
• Restricted interface no direct memory sharing
  between modules need to send messages via kernel
• Why good? Claimed advantages
• Extensibility new module new user space
  program/library
• Flexibility app can have own FS, Mem, Net, can
  make them fast
• Portability easier to port kernel to new
  hardware
• Reliability security each module has own
  protection domain. if crash, just restart cant
  affect other modules.

FS
Net
Mem
APP
User mode
kernel
Kernel mode
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Microkernel (cont.)

• Big thing in 90s best people worked on
  microkernel
• Students became top school professors
• Problem slow, too many user-kernel crossings
• Can be fixed with fast IPC
• However, there remain problems. In the end,
  either download extensions into kernel, or merge
  all modules into a library ? looks like
  monolithic kernels, maybe even more complicated!
• Today Windows, Linux, BSD, MacOS, all
  monolithic
• Some criticism on microkernel
• Restricted interface ? complicated implementation
• No shared state, hard to manage consistency
• Reliability security one key module fails,
  apps fail

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Modules

• Most microkernel advantages due to modularity
• Most modern operating systems implement kernel
  modules
• Uses object-oriented approach
• Function pointers in Linux strawman OOP with C
• Each talks to the others over known interfaces
• But share one protection domain, so just call
  function
• Each is loadable as needed within the kernel
• Overall, similar to microkernel, but more flexible

APP
User mode
FS
Net
Mem
Kernel mode
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Virtual Machine

• Virtual Machine Monitor (VMM) kernel that
  provides hardware interface
• Why good?
• Isolation. Strong protection between VMs
• Consolidation. One physical machine, multiple
  VMs
• Mobility. Can move VMs around
• Standardization same hw ? better system mgmt

APP
APP
APP
OS
OS
OS
User mode
Kernel mode
VMM
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Virtual Machine (cont)

• Normal operating system environment
• running in supervisor mode
• full access to machine state and I/O devices
• Virtualized guest operating systems
• running in user mode
• no direct access to machine state
• Tasks of the virtual machine monitor
• reconciling the virtual and physical architecture
• preventing virtual machines from interfering with
  each other or the monitor
• Do it fast? Not a easy job

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Hosted virtual machinesVMware Desktop Products
Architecture
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Today

• OS structure (cont.)
• Intro to Linux
• Interrupts in Linux
• System calls in Linux

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What is Linux?

• A modern, open-source OS based on UNIX standards
• 1991 written by Linus Torvalds from scratch, 0.1
  MLOC
• major design goal of UNIX compatibility
• Now many developers worldwide, 10 MLOC
• Unique management model
• Distributed development, central check in
• Linux distributions
• Ubuntu, Debian, Fedora, Redhat, CentOS,
  Slackware, Mandrake Linux, DreamLinux, SELinux,
  Gentoo,
• All based on the Linux kernel, with different set
  of applications, package management methods and
  configurations

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Linux Licensing

• The Linux kernel is distributed under the GNU
  General Public License (GPL), the terms of which
  are set out by the Free Software Foundation
• Anyone using Linux, or creating their own
  derivative of Linux, may not make the derived
  product proprietary software released under the
  GPL may not be redistributed as a binary-only
  product

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Linux kernel structure

• Core dynamically loadable modules
• Modules include device drivers, file systems,
  network protocols, etc
• Modules were originally developed to support the
  conditional inclusion of device drivers
• Early OS kernels would need to either
• include code for all possible devices or
• be recompiled to add support for a new device
• Now, Modules can be dynamically loaded and
  unloaded
• Modules are used extensively

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Linux kernel structure (cont.)
Applications
System Libraries (libc)
System Call Interface
I/O Related
Process Related
Scheduler
File Systems
Modules
Memory Management
Networking
IPC
Device Drivers
Architecture-Dependent Code
Hardware
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Linux source tree

• Download kernel.org (all releases revision
  history)
• Browse lxr.linux.no (with cross reference)
• Directory structure
• Public header files include/
• Each component is a subdir (e.g. mm/, ipc/
  driver/)
• Usually interface common functions loadable
  modules

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Today

• OS structure (cont.)
• Intro to Linux
• Interrupts in Linux
• How interrupts implemented Linux, using x86 as ex
• System calls in Linux

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Types of Interrupts on 80386

• Interrupts, asynchronous, from external devices,
  not related to code running
• Maskable interrupts
• Nonmaskable interrupts (NMI) hardware error
• Exceptions, synchronous, raised by CPU
• Processor-detected exceptions
• Faults correctable offending instruction is
  retried
• Traps often for debugging instruction is not
  retried
• Aborts major error (hardware failure), EIP
  wrong
• Programmed exceptions
• Requests for kernel intervention (software
  intr/syscalls)

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Faults

• Instruction would be illegal to execute
• Examples
• Writing to a memory segment marked read-only
• Reading from an unavailable memory segment (on
  disk) ? page fault
• Executing a privileged instruction
• Detected before incrementing the IP
• The causes of faults can often be fixed
• If a problem can be remedied, then the CPU can
  just resume its execution-cycle

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Traps

• A CPU might have been programmed to automatically
  switch control to a debugger program after it
  has executed an instruction
• That type of situation is known as a trap
• It is activated after incrementing the IP

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Handling Exceptions

• Most error exceptions divide by zero, invalid
  operation, illegal memory reference, etc.
  translate directly into signals
• This isnt a coincidence. . .
• The kernels job is fairly simple send the
  appropriate signal to the current process
• force_sig(sig_number, current)
• That will probably kill the process, but thats
  not the concern of the exception handler
• One important exception page fault
• An exception can (infrequently) happen in the
  kernel
• die() // kernel oops

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Interrupt assignment

• Total possible 0-255 Interrupt ID numbers
• First 32 reserved by Intel for NMI and exceptions
• OSs such as Linux are free to use the remaining
  224 available interrupt ID numbers for their own
  purposes (e.g., for service-requests from
  external devices, or for other purposes such as
  system-calls)
• Weve seen many examples in last lecture
• 0 divide-overflow fault
• 3 breakpoint
• 14 Page-Fault Exception
• 128 system call
• Called vector in ULK

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Interrupts in Linux
Memory Bus
IRQs
PIC
intr
idtr
CPU
IDT
INTR
0
intr
ISR
Assign IRQ to dev? IRQ to Intr ?
Mask points
255
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Assigning IRQs to Devices

• IRQ assignment is hardware-dependent
• Sometimes its hardwired, sometimes its set
  physically, sometimes its programmable
• PCI bus usually assigns IRQs at boot
• Some IRQs are fixed by the architecture
• IRQ0 Interval timer
• IRQ2 Cascade pin for 8259A
• Linux device drivers request IRQs when the device
  is opened
• Especially useful for dynamically-loaded drivers,
  such as for USB or PCMCIA devices
• Two devices that arent used at the same time can
  share an IRQ, even if the hardware doesnt
  support simultaneous sharing

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Assigning Interrupt to IRQs

• Intr index (0-255) into interrupt descriptor
  table
• Intr usually IRQ 32
• Below 32 reserved for non-maskable intr
  exceptions
• Maskable interrupts can be assigned as needed
• Vector 128 used for syscall
• Vectors 251-255 used for Inter-Processor
  Interrupt (IPI)

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Interrupts in Linux
Memory Bus
IRQs
PIC
intr
idtr
CPU
IDT
INTR
0
intr
ISR
Multicore?
Mask points
255
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Multiple Logical Processors
Multi-CORE CPU
CPU 0
CPU 1
I/O APIC
LOCAL APIC
LOCAL APIC
Advanced Programmable Interrupt Controller is
needed to perform routing of I/O requests
from peripherals to CPUs
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APIC, IO-APIC, LAPIC

• Advanced PIC (APIC) for SMP systems
• Used in all modern systems
• Interrupts routed to CPU over system bus
• IPI inter-processor interrupt
• Local APIC (LAPIC) versus frontend IO-APIC
• Devices connect to front-end IO-APIC
• IO-APIC communicates (over bus) with Local APIC
• Interrupt routing
• Allows broadcast or selective routing of
  interrupts
• Ability to distribute interrupt handling load
• Routes to lowest priority process
• Special register Task Priority Register (TPR)
• Arbitrates (round-robin) if equal priority

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Interrupts in Linux
Memory Bus
IRQs
PIC
intr
idtr
CPU
IDT
INTR
0
intr
ISR
How to set up IDT?
Mask points
255
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Interrupt Descriptor Table

• The entry-point to the interrupt-handler is
  located via the Interrupt Descriptor Table (IDT)
• IDT gate descriptors
• Location of handler
• Descriptor Privilege Level (DPL), prevent bad
  access
• Can invoke only when current privilege level
  (CPL) lt DPL
• This is just the mode bit for protection
• Gates (slightly different ways of entering
  kernel)
• Interrupt gate disables further interrupts
• Trap gate further interrupts still allowed
• Task gate includes TSS to transfer to (used when
  EIP is bad, or hardware failure)

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IDT Initialization

• Initialized once by BIOS in real mode
• Linux re-initializes during kernel init
• Must not expose kernel to user mode access
• start by setting all descriptors to null handler
  ignore_int()
• Then, set up entries we handle
• E.g. arch/i386/kernel/traps.c, function
  trap_init()

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Linux lingo

• Interrupt gate Intel Interrupt, maskable or non
  maskable
• no user access (DPL 0)
• disable interrupt when invoking handler
• E.g. set_intr_gate(2, nmi)
• System gate Intel trap with user access (DPL
  3) and interrupt enabled
• into (4), bounds (5), system call (128)
• E.g. set_system_gate(4, overflow)
• Sometimes want to disable interrupt for int3,
  set_system_interrupt_gate(3, int3)
• Trap gate Intel trap and fault, no user access
  (DPL 0) and interrupt enabled
• set_trap_gate(0, divide_error)

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Interrupts in Linux
Memory Bus
IRQs
PIC
intr
idtr
CPU
IDT
INTR
0
intr
ISR
How to load ISR?
Mask points
255
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Loading an Interrupt handler

• Hardware locates the proper gate descriptor for
  this interrupt vector, and locates the new
  context
• Verifies Current Privilege Level (CPL) lt
  Descriptor Privilege level (DPL)
• Load a new stack pointer if needed
• Hw saves old IP, etc on new stack
• Set IP, etc to interrupt handler invoke handler
• disable interrupt by unsetting IF bit in eflags
  register
• Handler saves old CPU state on new stack

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Finding the Proper Handler

• On modern hardware, multiple I/O devices can
  share a single IRQ and hence interrupt vector
• First differentiator is the interrupt vector
• Multiple interrupt service routines (ISR) can be
  associated with a vector
• Each devices ISR for that IRQ is called the
  determination of whether or not that device has
  interrupted is device-dependent

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Next lecture

• Interrupts in Linux (cont.)
• System calls in Linux
• Process (read OSC ch 3)