Handed out Wednesday, October 31, 2012
Due Thursday, November 8, 2012
In this lab, you will implement
spawn, a library call
that loads and runs on-disk executables.
You will then flesh out your kernel and library operating system
enough to run a shell on the console.
Use Git to fetch the latest version of the course repository, and then create a local branch called lab5 based on our lab5 branch, origin/lab5:
athena% cd ~/6.828/lab athena% add git athena% git pull Already up-to-date. athena% git checkout -b lab5 origin/lab5 Branch lab5 set up to track remote branch refs/remotes/origin/lab5. Switched to a new branch "lab5" athena% git merge lab4 Merge made by recursive. ..... athena%
The main new component for this part of the lab is the file system environment, located in the new fs directory. Scan through all the files in this directory to get a feel for what all is new. Also, there are some new file system-related source files in the user and lib directories,
|fs/fs.c||Code that mainipulates the file system's on-disk structure.|
|fs/bc.c||A simple block cache built on top of our user-level page fault handling facility.|
|fs/ide.c||Minimal PIO-based (non-interrupt-driven) IDE driver code.|
|fs/serv.c||The file system server that interacts with client environments using file system IPCs.|
|lib/fd.c||Code that implements the general UNIX-like file descriptor interface.|
|lib/file.c||The driver for on-disk file type, implemented as a file system IPC client.|
|lib/console.c||The driver for console input/output file type.|
|lib/spawn.c||Code skeleton of the spawn library call.|
You should run the pingpong, primes, and forktree test cases from lab
4 again after merging in the new lab 5 code. You will need to comment
ENV_CREATE(fs_fs) line in kern/init.c
because fs/fs.c tries to do some I/O, which JOS does not allow
Similarly, temporarily comment out the call to
lib/exit.c; this function calls subroutines that you will implement
later in the lab, and therefore will panic if called.
If your lab 4 code doesn't contain any bugs, the test cases should run
fine. Don't proceed until they work. Don't forget to un-comment
these lines when you start Exercise 1.
If they don't work, use git diff lab4 to review all the changes, making sure there isn't any code you wrote for lab4 (or before) missing from lab 5. Make sure that lab 4 still works.
As before, you will need to do all of the regular exercises described in the lab and at least one challenge problem. Additionally, you will need to write up brief answers to the questions posed in the lab and a short (e.g., one or two paragraph) description of what you did to solve your chosen challenge problem. If you implement more than one challenge problem, you only need to describe one of them in the write-up, though of course you are welcome to do more. Place the write-up in a file called answers-lab5.txt in the top level of your lab5 directory before handing in your work.
We have provided you with a simple, read-only, disk-based file system. You will need to slightly change your existing code in order to port the file system for your JOS, so that spawn can access on-disk executables using path names. Although you do not have to understand every detail of the file system, such as its on-disk structure. It is very important that you familiarize yourself with the design principles and its various interfaces.
The file system itself is implemented in micro-kernel fashion, outside the kernel but within its own user-space environment. Other environments access the file system by making IPC requests to this special file system environment.
The file system environment in our operating system needs to be able to access the disk, but we have not yet implemented any disk access functionality in our kernel. Instead of taking the conventional "monolithic" operating system strategy of adding an IDE disk driver to the kernel along with the necessary system calls to allow the file system to access it, we instead implement the IDE disk driver as part of the user-level file system environment. We will still need to modify the kernel slightly, in order to set things up so that the file system environment has the privileges it needs to implement disk access itself.
It is easy to implement disk access in user space this way as long as we rely on polling, "programmed I/O" (PIO)-based disk access and do not use disk interrupts. It is possible to implement interrupt-driven device drivers in user mode as well (the L3 and L4 kernels do this, for example), but it is more difficult since the kernel must field device interrupts and dispatch them to the correct user-mode environment.
The x86 processor uses the IOPL bits in the EFLAGS register to determine whether protected-mode code is allowed to perform special device I/O instructions such as the IN and OUT instructions. Since all of the IDE disk registers we need to access are located in the x86's I/O space rather than being memory-mapped, giving "I/O privilege" to the file system environment is the only thing we need to do in order to allow the file system to access these registers. In effect, the IOPL bits in the EFLAGS register provides the kernel with a simple "all-or-nothing" method of controlling whether user-mode code can access I/O space. In our case, we want the file system environment to be able to access I/O space, but we do not want any other environments to be able to access I/O space at all.
i386_init identifies the file system environment by
passing the type
ENV_TYPE_FS to your environment creation
env_create in env.c,
so that it gives the file system environment I/O privilege,
but never gives that privilege to any other environment.
Make sure you can start the file environment without causing a General Protection fault. You should pass the "fs i/o" test in make grade.
Note that the GNUmakefile file in this lab sets up QEMU to use the file obj/kern/kernel.img as the image for disk 0 (typically "Drive C" under DOS/Windows) as before, and to use the (new) file obj/fs/fs.img as the image for disk 1 ("Drive D"). In this lab our file system should only ever touch disk 1; disk 0 is used only to boot the kernel.
In our file system, we will implement a simple "buffer cache" (really just a block cache) with the help of the processor's virtual memory system. The code for the block cache is in fs/bc.c.
Our file system will be limited to handling disks of size 3GB or less.
We reserve a large, fixed 3GB region
of the file system environment's address space,
from 0x10000000 (
up to 0xD0000000 (
as a "memory mapped" version of the disk.
disk block 0 is mapped at virtual address 0x10000000,
disk block 1 is mapped at virtual address 0x10001000,
and so on. The
diskaddr function in fs/bc.c
implements this translation from disk block numbers to virtual
addresses (along with some sanity checking).
Since our file system environment has its own virtual address space independent of the virtual address spaces of all other environments in the system, and the only thing the file system environment needs to do is to implement file access, it is reasonable to reserve most of the file system environment's address space in this way. It would be awkward for a real file system implementation on a 32-bit machine to do this since modern disks are larger than 3GB. Such a buffer cache management approach may still be reasonable on a machine with a 64-bit address space.
Of course, it would be unreasonable to read the entire disk into memory, so instead we'll implement a form of demand paging, wherein we only allocate pages in the disk map region and read the corresponding block from the disk in response to a page fault in this region. This way, we can pretend that the entire disk is in memory.
bc_pgfault functions in fs/bc.c.
bc_pgfault is a page fault handler, just like the one
your wrote in the previous lab for copy-on-write fork, except that
its job is to load pages in from the disk in response to a page
fault. When writing this, keep in mind that (1)
may not be aligned to a block boundary and (2)
operates in sectors, not blocks.
Use make grade to test your code. Your code should pass "check_super".
fs_initfunction in fs/fs.c is a prime example of how to use the block cache. After initializing the block cache, it simply stores pointers into the disk map region in the
superglobal variable. After this point, we can simply read from the
superstructure as if they were in memory and our page fault handler will read them from disk as necessary.
Now that we have the necessary functionality within the file system environment itself, we must make it accessible to other environments that wish to use the file system. Since other environments can't directly call functions in the file system environment, we'll expose access to the file system environment via a remote procedure call, or RPC, abstraction, built atop JOS's IPC mechanism. Graphically, here's what a call to the file system server (say, read) looks like
Regular env FS env +---------------+ +---------------+ | read | | file_read | | (lib/fd.c) | | (fs/fs.c) | ...|.......|.......|...|.......^.......|............... | v | | | | RPC mechanism | devfile_read | | serve_read | | (lib/file.c) | | (fs/serv.c) | | | | | ^ | | v | | | | | fsipc | | serve | | (lib/file.c) | | (fs/serv.c) | | | | | ^ | | v | | | | | ipc_send | | ipc_recv | | | | | ^ | +-------|-------+ +-------|-------+ | | +-------------------+
Everything below the dotted line is simply the mechanics of getting a
read request from the regular environment to the file system
environment. Starting at the beginning,
read (which we
provide) works on any file descriptor and simply dispatches to the
appropriate device read function, in this case
devfile_read (we can have more device types, like pipes).
read specifically for on-disk files. This and
devfile_* functions in lib/file.c
implement the client side of the FS operations and all work in roughly
the same way, bundling up arguments in a request structure, calling
fsipc to send the IPC request, and unpacking and
returning the results. The
fsipc function simply handles
the common details of sending a request to the server and receiving
The file system server code can be found in fs/serv.c. It
loops in the
serve function, endlessly receiving a
request over IPC, dispatching that request to the appropriate handler
function, and sending the result back via IPC. In the read example,
serve will dispatch to
will take care of the IPC details specific to read requests such as
unpacking the request structure and finally call
file_read to actually perform the file read.
Recall that JOS's IPC mechanism lets an environment send a single
32-bit number and, optionally, share a page. To send a request from
the client to the server, we use the 32-bit number for the request
type (the file system server RPCs are numbered, just like how
syscalls were numbered) and store the arguments to the request in a
union Fsipc on the page shared via the IPC. On the
client side, we always share the page at
fsipcbuf; on the
server side, we map the incoming request page at
The server also sends the response back via IPC. We use the 32-bit
number for the function's return code. For most RPCs, this is all
return data, which they simply write to the page that the client sent
its request on. There's no need to send this page in the response
IPC, since the client shared it with the file system server in the
first place. Also, in its response,
FSREQ_OPEN shares with
the client a new "Fd page". We'll return to the file descriptor
Challenge! Extend the file system to support write access. Here are a few points you need to consider:
file_get_block, allocate new disk blocks as necessary.
PTE_D"dirty" bit in the
uvptentry) to keep track of whether a cached disk block has been modified, and thus needs to be written back to the disk.
O_TRUNCopen modes in
FSREQ_SYNC, in fs/serv.c. We have defined the argument for these calls for you in inc/fs.h. Also, write the corresponding service routines in fs/fs.c and hook them to client stubs in lib/file.c.
We have given you the code for
which creates a new environment,
loads a program image from the file system into it,
and then starts the child environment running this program.
The parent process then continues running independently of the child.
spawn function effectively acts like a
fork in UNIX
followed by an immediate
exec in the child process.
spawn rather than a
spawn is easier to
implement from user space in "exokernel fashion", without special help
from the kernel. Think about what you would have to do in order to
exec in user space, and be sure you understand
why it is harder.
spawn relies on the new syscall
sys_env_set_trapframe to initialize the state of the
newly created environment. Implement
sys_env_set_trapframe. Test your code by running the
from kern/init.c, which will attempt to
spawn /hello from the file system.
Use make grade to test your code.
mmap-style memory-mapped files and
spawn to map pages directly from the ELF
image when possible.
The UNIX file descriptors are a general notion that also
encompasses pipes, console I/O, etc. In JOS, each of these device
types has a corresponding
with pointers to the functions that implement
read/write/etc. for that device type. lib/fd.c implements the
general UNIX-like file descriptor interface on top of this. Each
struct Fd indicates its device type, and most of the
functions in lib/fd.c simply dispatch operations to functions
in the appropriate
lib/fd.c also maintains the file descriptor table
region in each application environment's address space, starting at
FSTABLE. This area reserves a page's worth (4KB) of
address space for each of the up to
MAXFD (currently 32)
file descriptors the application can have open at once. At any given
time, a particular file descriptor table page is mapped if and only if
the corresponding file descriptor is in use. Each file descriptor
also has an optional "data page" in the region starting at
FILEDATA, which devices can use if they choose.
We would like to share file descriptor state across
spawn, but file descriptor state is kept
in user-space memory. Right now, on
fork, the memory
will be marked copy-on-write,
so the state will be duplicated rather than shared.
(This means environments won't be able to seek in files they
didn't open themselves and that pipes won't work across a fork.)
spawn, the memory will be
left behind, not copied at all. (Effectively, the spawned environment
starts with no open file descriptors.)
We will change
fork to know that
certain regions of memory are used by the "library operating system" and
should always be shared. Rather than hard-code a list of regions somewhere,
we will set an otherwise-unused bit in the page table entries (just like
we did with the
PTE_COW bit in
We have defined a new
This bit is one of the three PTE bits
that are marked "available for software use"
in the Intel and AMD manuals.
We will establish the convention that
if a page table entry has this bit set,
the PTE should be copied directly from parent to child
Note that this is different from marking it copy-on-write:
as described in the first paragraph,
we want to make sure to share
updates to the page.
duppage in lib/fork.c to follow
the new convention. If the page table entry has the
bit set, just copy the mapping directly.
(You should use
to mask out the relevant bits from the page table entry.
picks up the accessed and dirty bits as well.)
lib/spawn.c. It should loop through all page table
entries in the current process (just like
did), copying any page mappings that have the
PTE_SHARE bit set into the child process.
Use make run-testpteshare to check that your code is behaving properly. You should see lines that say "fork handles PTE_SHARE right" and "spawn handles PTE_SHARE right".
Use make run-testfdsharing to check that file descriptors are shared properly. You should see lines that say "read in child succeeded" and "read in parent succeeded".
For the shell to work, we need a way to type at it. QEMU has been displaying output we write to the CGA display and the serial port, but so far we've only taken input while in the kernel monitor. In QEMU, input typed in the graphical window appear as input from the keyboard to JOS, while input typed to the console appear as characters on the serial port. kern/console.c already contains the keyboard and serial drivers that have been used by the kernel monitor since lab 1, but now you need to attach these to the rest of the system.
In your kern/trap.c, call
kbd_intr to handle trap
We implemented the console input/output file type for you, in lib/console.c.
Test your code by running make run-testkbd and type a few lines. The system should echo your lines back to you as you finish them. Try typing in both the console and the graphical window, if you have both available.
Run make run-icode or make run-icode-nox. This will run your kernel and start user/icode. icode execs init, which will set up the console as file descriptors 0 and 1 (standard input and standard output). It will then spawn sh, the shell. You should be able to run the following commands:
echo hello world | cat cat lorem |cat cat lorem |num cat lorem |num |num |num |num |num lsfd cat script sh <script
Note that the user library routine
to the console, without using the file descriptor code. This is great
for debugging but not great for piping into other programs.
To print output to a particular file descriptor (for example, 1, standard output),
fprintf(1, "...", ...).
printf("...", ...) is a short-cut for printing to FD 1.
See user/lsfd.c for examples.
Run make run-testshell to test your shell. testshell simply feeds the above commands (also found in fs/testshell.sh) into the shell and then checks that the output matches fs/testshell.key.
Your code should pass all tests at this point. As usual, you can grade your submission with make grade and hand it in with make handin.
This completes the lab. As usual, don't forget to run make grade and to write up your answers and a description of your challenge exercise solution. Before handing in, use git status and git diff to examine your changes and don't forget to git add answers-lab5.txt. When you're ready, commit your changes with git commit -am 'my solutions to lab 5', then make handin to submit your solution.