Fall 2017 :: CSE 306 — Operating Systems

Introduction

Familiarity with your environment is crucial for productive development and debugging. This page gives a brief overview of the xv6 environment and useful GDB and QEMU commands. Don't take our word for it, though. Read the GDB and QEMU manuals. These are powerful tools that are worth knowing how to use.

All of the tools that you need to execute and complete the labs are installed in your course virtual machine.

If you wish to compile and run the tools on your own machine, the information that you need is as follows. Note that we cannot guarantee that these tools will run on your computer, and we cannot support these tools on your own computer. However, the tools should run on recent versions of Linux.

Compiler Toolchain

Most modern Linux distributions and BSDs have an ELF toolchain compatible with xv6 labs. That is, the system-standard gcc, as, ld and objdump should just work. The lab Makefile should automatically detect this. If the makefile fails to detect your build tools, you can specify their location by setting the TOOLPREFIX variable in the makefile.

GDB

The standard version of GDB has enough support to let you debug your xv6 code. You can either compile it from source or install it using common package managers such as apt, yumm, etc.

QEMU Emulator

QEMU is a modern and fast PC emulator. The standard version of QEMU has enough support for xv6 labs. You can either compile it from source or install it using common package managers such as apt, yumm, etc.

Debugging tips

Kernel

GDB is your friend. Use the qemu-gdb target (or its qemu-gdb-nox variant) to make QEMU wait for GDB to attach. See the GDB reference below for some commands that are useful when debugging kernels.

If you're getting unexpected interrupts, exceptions, or triple faults, you can ask QEMU to generate a detailed log of interrupts using the -d argument.

To debug virtual memory issues, try the QEMU monitor commands info mem (for a high-level overview) or info pg (for lots of detail). Note that these commands only display the current page table.

Reference

xv6 makefile

The xv6 makefile includes a number of phony targets for running xv6 in various ways. All of these targets configure QEMU to listen for GDB connections (the *-gdb targets also wait for this connection). To start once QEMU is running, simply run gdb from your lab directory. We provide a .gdbinit file that automatically points GDB at QEMU, loads the kernel symbol file, and switches between 16-bit and 32-bit mode. Exiting GDB will shut down QEMU.
make qemu
Build everything and start QEMU with the VGA console in a new window and the serial console in your terminal. To exit, either close the VGA window or press Ctrl-c or Ctrl-a x in your terminal.
make qemu-nox
Like make qemu, but run with only the serial console. To exit, press Ctrl-a x. This is particularly useful over SSH connections because the VGA window consumes a lot of bandwidth.
make qemu-gdb
Like make qemu, but rather than passively accepting GDB connections at any time, this pauses at the first machine instruction and waits for a GDB connection.
make qemu-nox-gdb
A combination of the qemu-nox and qemu-gdb targets.

GDB

See the GDB manual for a full guide to GDB commands. Here are some particularly useful commands for this course, some of which don't typically come up outside of OS development.

Ctrl-c
Halt the machine and break in to GDB at the current instruction. If QEMU has multiple virtual CPUs, this halts all of them.
c (or continue)
Continue execution until the next breakpoint or Ctrl-c.
si (or stepi)
Execute one machine instruction.
b function or b file:line (or breakpoint)
Set a breakpoint at the given function or line.
b *addr (or breakpoint)
Set a breakpoint at the EIP addr.
set print pretty
Enable pretty-printing of arrays and structs.
info registers
Print the general purpose registers, eip, eflags, and the segment selectors. For a much more thorough dump of the machine register state, see QEMU's own info registers command.
x/Nx addr
Display a hex dump of N words starting at virtual address addr. If N is omitted, it defaults to 1. addr can be any expression.
x/Ni addr
Display the N assembly instructions starting at addr. Using $eip as addr will display the instructions at the current instruction pointer.
symbol-file file
(Lab 3+) Switch to symbol file file. When GDB attaches to QEMU, it has no notion of the process boundaries within the virtual machine, so we have to tell it which symbols to use. By default, we configure GDB to use the kernel symbol file, obj/kern/kernel. If the machine is running user code, say hello.c, you can switch to the hello symbol file using symbol-file obj/user/hello.

QEMU represents each virtual CPU as a thread in GDB, so you can use all of GDB's thread-related commands to view or manipulate QEMU's virtual CPUs.

thread n
GDB focuses on one thread (i.e., CPU) at a time. This command switches that focus to thread n, numbered from zero.
info threads
List all threads (i.e., CPUs), including their state (active or halted) and what function they're in.

QEMU

QEMU includes a built-in monitor that can inspect and modify the machine state in useful ways. To enter the monitor, press Ctrl-a c in the terminal running QEMU. Press Ctrl-a c again to switch back to the serial console.

For a complete reference to the monitor commands, see the QEMU manual. Here are some particularly useful commands:

xp/Nx paddr
Display a hex dump of N words starting at physical address paddr. If N is omitted, it defaults to 1. This is the physical memory analogue of GDB's x command.
info registers
Display a full dump of the machine's internal register state. In particular, this includes the machine's hidden segment state for the segment selectors and the local, global, and interrupt descriptor tables, plus the task register. This hidden state is the information the virtual CPU read from the GDT/LDT when the segment selector was loaded. Here's an example and the meaning of each field: 
CS =0008 10000000 ffffffff 10cf9a00 DPL=0 CS32 [-R-]
CS =0008
The visible part of the code selector. We're using segment 0x8. This also tells us we're referring to the global descriptor table (0x8&4=0), and our CPL (current privilege level) is 0x8&3=0.
10000000
The base of this segment. Linear address = logical address + 0x10000000.
ffffffff
The limit of this segment. Linear addresses above 0xffffffff will result in segment violation exceptions.
10cf9a00
The raw flags of this segment, which QEMU helpfully decodes for us in the next few fields.
DPL=0
The privilege level of this segment. Only code running with privilege level 0 can load this segment.
CS32
This is a 32-bit code segment. Other values include DS for data segments (not to be confused with the DS register), and LDT for local descriptor tables.
[-R-]
This segment is read-only.
info mem
Display mapped virtual memory and permissions. For example, 
ef7c0000-ef800000 00040000 urw
efbf8000-efc00000 00008000 -rw
tells us that the 0x00040000 bytes of memory from 0xef7c0000 to 0xef800000 are mapped read/write and user-accessible, while the memory from 0xefbf8000 to 0xefc00000 is mapped read/write, but only kernel-accessible.
QEMU also takes some useful command line arguments, which can be passed into the xv6 makefile using the QEMUEXTRA variable.
make QEMUEXTRA='-d int' ...
Log all interrupts, along with a full register dump, to qemu.log. You can ignore the first two log entries, "SMM: enter" and "SMM: after RMS", as these are generated before entering the boot loader. After this, log entries look like 
     4: v=30 e=0000 i=1 cpl=3 IP=001b:00800e2e pc=00800e2e SP=0023:eebfdf28 EAX=00000005
EAX=00000005 EBX=00001002 ECX=00200000 EDX=00000000
ESI=00000805 EDI=00200000 EBP=eebfdf60 ESP=eebfdf28
...
The first line describes the interrupt. The 4: is just a log record counter. v gives the vector number in hex. e gives the error code. i=1 indicates that this was produced by an int instruction (versus a hardware interrupt). The rest of the line should be self-explanatory. See info registers for a description of the register dump that follows.