6.828 lab tools guide
Familiarity with your environment is crucial for productive
development and debugging. This page gives a brief overview of the
JOS 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.
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
(Lab 4+) To debug multiple CPUs, use GDB's thread-related commands
like thread and info threads.
User environments (lab 3+)
GDB also lets you debug user environments, but there are a few
things you need to watch out for, since GDB doesn't know that there's
a distinction between multiple user environments, or between user and
You can start JOS with a specific user environment using make run-name (or you can edit
kern/init.c directly). To make QEMU wait for GDB to attach,
use the run-name-gdb
You can symbolically debug user code, just like you can kernel
code, but you have to tell GDB which symbol
table to use with the symbol-file command, since it
can only use one symbol table at a time. The provided
.gdbinit loads the kernel symbol table,
obj/kern/kernel. The symbol table for a user environment is
in its ELF binary, so you can load it using symbol-file
obj/user/name. Don't load symbols from any
.o files, as those haven't been relocated by the linker
(libraries are statically linked into JOS user binaries, so those
symbols are already included in each user binary). Make sure you get
the right user binary; library functions will be linked at
different EIPs in different binaries and GDB won't know any
(Lab 4+) Since GDB is attached to the virtual machine as a whole,
it sees clock interrupts as just another control transfer. This makes
it basically impossible to step through user code because a clock
interrupt is virtually guaranteed the moment you let the VM run again.
The stepi command works because it
suppresses interrupts, but it only steps one assembly instruction. Breakpoints generally work, but watch out because
you can hit the same EIP in a different environment (indeed, a
different binary altogether!).
The JOS GNUmakefile includes a number of phony targets for running JOS
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
The makefile also accepts a few useful variables:
- 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 to Athena dialups 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
- make run-name
- (Lab 3+) Run user program name. For example, make
run-hello runs user/hello.c.
- make run-name-nox,
- (Lab 3+) Variants of run-name that correspond to
the variants of the qemu target.
- make V=1 ...
- Verbose mode. Print out every command being executed, including
- make V=1 grade
- Stop after any failed grade test and leave the QEMU output in
jos.out for inspection.
- Specify additional arguments to pass to QEMU.
When building JOS, the makefile also produces some additional
output files that may prove useful while debugging:
obj/kern/kernel.asm, obj/user/hello.asm, etc.
- Assembly code listings for the bootloader, kernel, and user
- Symbol tables for the kernel and user programs.
- obj/boot/boot.out, obj/kern/kernel,
- Linked ELF images of the kernel and user programs. These
contain symbol information that can be used by GDB.
See the GDB
manual for a full guide to GDB commands. Here are some
particularly useful commands for 6.828, some of which don't typically
come up outside of OS development.
- Halt the machine and break in to GDB at the current
instruction. If QEMU has multiple virtual CPUs, this halts all of
- 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
- 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
- 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
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
- 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 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:
QEMU also takes some useful command line arguments, which can be
passed into the JOS makefile using the QEMUEXTRA variable.
- 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
- 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 the CS when running in the JOS kernel
in lab 1 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
- The base of this segment. Linear address = logical address +
- The limit of this segment. Linear addresses above 0xffffffff
will result in segment violation exceptions.
- The raw flags of this segment, which QEMU helpfully decodes
for us in the next few fields.
- The privilege level of this segment. Only code running with
privilege level 0 can load this segment.
- 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.
- This segment is read-only.
- info mem
- (Lab 2+) Display mapped virtual memory and permissions. For
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
- info pg
- (Lab 2+) Display the current page table structure. The output
is similar to info mem, but distinguishes page directory
entries and page table entries and gives the permissions for each
separately. Repeated PTE's and entire page tables are folded up
into a single line. For example,
VPN range Entry Flags Physical page
[00000-003ff] PDE -------UWP
[00200-00233] PTE[200-233] -------U-P 00380 0037e 0037d 0037c 0037b 0037a ..
[00800-00bff] PDE ----A--UWP
[00800-00801] PTE[000-001] ----A--U-P 0034b 00349
[00802-00802] PTE -------U-P 00348
This shows two page directory entries, spanning virtual addresses
0x00000000 to 0x003fffff and 0x00800000 to 0x00bfffff, respectively.
Both PDE's are present, writable, and user and the second PDE is also
accessed. The second of these page tables maps three pages, spanning
virtual addresses 0x00800000 through 0x00802fff, of which the first
two are present, user, and accessed and the third is only present and
user. The first of these PTE's maps physical page 0x34b.
- 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.
- Note: If you're running a pre-0.15 version of QEMU, the log will
be written to /tmp instead of the current directory.