| Part 0: Introduction |
In this lab, your will write the memory management code for your operating system. Memory management is comprised of two components.
The first component that comes under the umbrella of memory management is virtual memory, where we set up the PC'S Memory Management Unit (MMU) hardware to map the virtual addresses used by software into different, physical addresses for purposes of accessing memory. You will set up the virtual memory layout for JOS according to the specification we provide. Your task will be to build a page table data structure to match our specification.
The second component is managing the physical memory of the computer so that the kernel can be dynamically allocate memory for various uses, and later deallocate that memory and re-assign it for different purposes. The x86 divides physical memory up into 4096 byte regions called pages. Your task will be to maintain data structures that record which pages are free and allocated and how many processes are sharing each allocated page. You will also write the routines to allocate and free pages of memory.
Download the code for lab 2 from http://www.cs.utexas.edu/users/lorenzo/corsi/cs372h/07S/labs/lab2/lab2.tar.gz and untar it into your 372 directory, just as you did for lab 1. You will then need to merge the changes between our lab 1 and lab 2 source code trees into your own kernel code resulting from completing lab 1.
In this and future labs you will progressively build on this same kernel. With each new lab we will hand out a source tree containing additional files and possibly some changes to existing files. You will need to compare the new source tree against the one we provided for the previous lab in order to figure out what new code you need to incorporate into your kernel. You may find it useful to keep a "pristine" copy of our source tree for each lab around along with your modified versions. You should expect to become intimately familiar with the Unix diff utility if you aren't already, and patch can be highly useful as well. If you're particularly organized you might try using cvs and learn how to deal with branches. "Diff-and-merge" is an important and unavoidable component of all real OS development activity, so any time you spend learning to do this effectively is time well spent.
One option is to just merge in your changes manually. If you remember what functions you modified, you can copy the changes into the lab2 code. To actually see what changes you made, and try to patch them in to the code, run the following sequence of commands. Be warned that these utilities are not perfect, and merging in the changes by hand may be simpler.
cd ~/372 # this creates a tar of what you handed in, for backup purposes tar czvf lab1-handin.tar.gz lab1 mkdir given-code cd given-code tar xzf ../lab1.tar.gz cd .. mv given-code/lab1 lab1-unchanged # now we have the handed out lab1 code in lab1-unchanged diff -r -u lab1-unchanged lab1 > lab1-changes.txt # It is very important to look at the patch file. All of the changes # in it should be for code that you added to lab 1 and want to bring # to lab 2. If there are other changes (like changes to the # makefiles), then you should NOT run the 'patch' command below. # Instead, you should apply the patch by hand. If you decide to apply # it with patch, then run the commands below. cd lab2 patch -p1 -u < ../lab1-changes.txt # if any chunks failed, then you will need to look at the rejects # files (.rej) and merge those changes in yourself.
Lab 2 contains the following new source files, which you should browse through as you merge them into your kernel:
memlayout.h describes the layout of the virtual address space that you must implement by modifying pmap.c, memlayout.h, and pmap.h define the Page structure that you'll use to keep track of which pages of physical memory are free. kclock.c and kclock.h manipulate the PC's battery-backed clock and CMOS RAM hardware, in which the BIOS records the amount of physical memory the PC contains, among other things. The code in pmap.c needs to read this device hardware in order to figure out how much physical memory there is, but that part of the code is done for you: you do not need to know the details of how the CMOS hardware works. The last two files provide support for extending the kernel monitor.
At this point, the code should compile (with some warnings that you will fix) and the kernel should boot and then give a kernel panic ``i386_vm_init: This function is not finished''
| Part 1: Virtual Memory |
Before doing anything else, you will need to familiarize yourself with the x86's protected-mode memory management architecture: namely segmentation and page translation.
| Exercise 1. Read chapters 5 and 6 of the Intel 80386 Reference Manual, if you haven't done so already. Although JOS relies most heavily on page translation, you will also need a basic understanding of how segmentation works in protected mode to understand what's going on in JOS. |
In x86 terminology, a virtual address is a "segment:offset"-style address before segment translation is performed; a linear address is what you get after segmentation but before page translation; and a physical address is what you finally get after both segmentation and page translation. Be sure you understand the difference between these three types or "levels" of addresses!
The JOS kernel tries to use consistent type names for different kinds of address. In particular, the type uintptr_t represents virtual addresses, and physaddr_t represents physical addresses. Of course, both these types are really just synonyms for 32-bit integers (uint32_t), so the compiler won't stop you from assigning one type to another! Every pointer value in JOS should be a virtual address (once paging is set up), since only virtual addresses can be dereferenced. The kernel runs in protected mode too! To summarize:
| C type | Address type |
| T* | Virtual |
| uintptr_t | Virtual |
| physaddr_t | Physical |
Exercise 2.
Review the
debugger section in the
Bochs user manual,
and make sure you understand which debugger commands
deal with which kinds of addresses.
In particular, note the various vb, lb,
and pb breakpoint commands to set breakpoints at
virtual, linear, and physical addresses.
The default b command breaks at a physical address.
Also note that the x command
examines data at a linear address,
while the command xp takes a physical address.
Sadly there is no xv at all.
Examine |
Exercise 3.
Fortunately, we don't have to rely entirely on low-level Bochs degugging
this year. Read about using Bochs with GDB. A version of Bochs with
GDB enabled is at /p/graft/bochs-2.3/bochs-gdb. Update your .bochsrc
as described in the instructions, run bochs-gdb
in one window, and run gdb (or ddd) in another window. Using gdb (or ddd),
put a breakpoint at the start of the monitor()
function and run the
kernel to verify that the breakpoint works. Step through a few
statements and get familiar with the symbolic debugger.
Unfortunately, you will still need to make use of the low-level Bochs debugger for some parts of the lab such as when we are doing low-level interrupt handling routines and when we are debugging user-space programs that run under our kernel. Note: If you get the gdb warning "Reply contains invalid hex digit 78", it appears to be harmless to ignore it. |
In Part 3 of Lab 1 we noted that the boot loader sets up the x86 segmentation hardware so that the kernel appears to run at its link address of 0xf0100000, even though it is actually loaded in physical memory just above the ROM BIOS at 0x00100000. In other words, the kernel's virtual starting address at this point is 0xf0100000, but its linear and physical starting addresses are both 0x00100000. The kernel's linear and physical addresses are the same because we have not yet initialized or enabled page translation.
In the virtual memory layout you are going to set up for JOS, we will stop using the x86 segmentation hardware for anything interesting, and instead start using page translation to accomplish everything we've already done with segmentation and much more. That is, after you finish this lab and the JOS kernel successfully enables paging, linear addresses will be the same as (the offset portion of) the kernel's virtual addresses, rather than being the same as physical addresses as they are when the boot loader first enters the kernel.
In JOS,
we divide the processor's 32-bit linear address space
into two parts.
User environments (processes),
which we will begin loading and running in lab 3,
will have control over the layout and contents of the lower part,
while the kernel always maintains complete control over the upper part.
The dividing line is defined somewhat arbitrarily
by the symbol ULIM in inc/memlayout.h,
reserving approximately 256MB of linear (and therefore virtual) address space
for the kernel.
This explains why we needed to give the kernel
such a high link address in lab 1:
otherwise there would not be enough room in the kernel's linear address space
to map in a user environment below it at the same time.
Since the kernel and user environment will effectively co-exist in each environment's address space, we will have to use permission bits in our x86 page tables to prevent user code from accessing the kernel's memory: i.e., to enforce fault isolation. We do this as follows.
The user environment will have no permission to any of the
memory above ULIM, while the kernel will be able to
read and write this memory. For the address range
(UTOP,ULIM], both the kernel and the user environment have
the same permission: they can read but not write this address range.
This range of address is used to expose certain kernel data structures
read-only to the user environment. Lastly, the address space below
UTOP is for the user environment to use; the user environment
will set permissions for accessing this memory.
In this lab, you are going to set up the address space above
UTOP - the kernel part of the address space.
The layout of this portion of the virtual address space will be
handled by the i386_vm_init() function, defined in
kern/pmap.c. The actual layout is as described
is diagrammed in inc/memlayout.h. It would behoove you to
become familiar with this file
as well as inc/mmu.h,
which contains useful macros and definitions
relating to the x86 memory management hardware.
This would also be a really good time to learn to use etags (for emacs, or the equivalent in your favorite editor) to jump to any symbol you specify. Tools like this are essential for making sense of large bodies of code.
Exercise 4.
Implement the following functions in kern/pmap.c:
boot_alloc() boot_pgdir_walk() boot_map_segment() i386_vm_init()The comments in i386_vm_init() specify the virtual memory
layout. The other functions provide useful
abstractions for your implementation of i386_vm_init().
Your task is to fill in the missing code to build a 2-level
page table fulfilling this specification.
The other functions are helper routines you will find useful.
Once you have done this, run the code. The function call to
Warning! This check is far from comprehensive; in previous years, a few groups passed this check even though their code had bugs. These groups discovered in a later lab (and via much pain) that their basic memory management code was broken. You are strongly advised to add your own tests to both parts of the lab. Hint. Rather than blindly running the tests, use gdb to to step through them to see if they make sense. Hint. Don't forget to properly set permission in both the PDE and PTE. |
Exercise 5.
Answer the following questions in the lab's README file.
|
Challenge!
Extend the JOS kernel monitor with commands to:
|
The address space layout we use in JOS is not the only one possible. An operating system might map the kernel at low linear addresses while leaving the upper part of the linear address space for user processes. x86 kernels generally do not take this approach, however, because one of the x86's backward-compatibility modes, known as virtual 8086 mode, is "hard-wired" in the processor to use the bottom part of the linear address space, and thus cannot be used at all if the kernel is mapped there.
It is even possible, though much more difficult, to design the kernel so as not to have to reserve any fixed portion of the processor's linear or virtual address space for itself, but instead effectively to allow allow user-level processes unrestricted use of the entire 4GB of virtual address space - while still fully protecting the kernel from these processes and protecting different processes from each other!
| Challenge! Write up an outline of how a kernel could be designed to allow user environments unrestricted use of the full 4GB virtual and linear address space. Hint: the technique is sometimes known as "follow the bouncing kernel." In your design, be sure to address exactly what has to happen when the processor transitions between kernel and user modes, and how the kernel would accomplish such transitions. Also describe how the kernel would access physical memory and I/O devices in this scheme, and how the kernel would access a user environment's virtual address space during system calls and the like. Finally, think about and describe the advantages and disadvantages of such a scheme in terms of flexibility, performance, kernel complexity, and other factors you can think of. |
| Part 2: Physical Page Management |
Besides setting up the processor hardware to translate virtual addresses correctly into physical addresses, the operating system must also keep track of which parts of available RAM are free and which are currently in use for various purposes. In JOS we will manage the PC's physical memory strictly on a page granularity: i.e., only in units of whole, page-aligned 4KB pages. This design simplifies the memory management system and nicely matches the 4KB page size that the processor uses for page translation purposes.
Exercise 6.
In the file kern/pmap.c,
you must implement code for
the functions listed below: You may find it useful
to read inc/memlayout.h and kern/pmap.h.
page_init()
page_alloc()
page_free()
pgdir_walk()
page_insert()
page_lookup()
page_remove()
The function page_check(),
called from i386_init(),
tests these functions.
You must get page_check() to run successfully.
Hint! Data structures like
|
Exercise 7.
Answer the following questions in the README file.
|
|
Challenge! Modify your stack backtrace
function to display, for each EIP, the function name,
source file name, and line number corresponding to that EIP.
To help you we have provided
debuginfo_eip, which looks up eip in
the symbol table and is defined in kern/kdebug.c.
In debuginfo_eip, where do __STAB_* come from? This question has a long answer; to help you to discover the answer, here are some things you might want to do:
Complete the implementation of debuginfo_eip by inserting the call to stab_binsearch to find the line number for an address. Extend your implementation of mon_backtrace to call debuginfo_eip and print a line for each stack frame of the form: Stack backtrace: kern/monitor.c:74: mon_backtrace+10 ebp f0119ef8 eip f01008ce args 00000001 f0119f20 00000000 00000000 2000000a kern/monitor.c:143: monitor+10a ebp f0119f78 eip f01000e5 args 00000000 f0119fac 00000275 f01033cc fffffffc kern/init.c:78: _panic+51 ebp f0119f98 eip f010133e args f01033ab 00000275 f01033cc f0103473 f01030bc kern/pmap.c:711: page_check+9e ebp f0119fd8 eip f0100082 args f0102d20 00001aac 000006a0 00000000 00000000 kern/init.c:36: i386_init+42 ebp f0119ff8 eip f010003d args 00000000 00000000 0000ffff 10cf9a00 0000ffffThe read_eip() function may help with the first line. You may find that the some functions are missing from the backtrace. For example, you will probably see a call to monitor() but not to runcmd(). This is because the compiler in-lines some function calls. Other optimizations may cause you to see unexpected line numbers. If you get rid of the -O2 from GNUMakefile, the backtraces may make more sense (but your kernel will run more slowly). |
|
Challenge!
We consumed many physical pages to hold the
page tables for the KERNBASE mapping.
Do a more space-efficient job using the PTE_PS ("Page Size") bit
in the page directory entries.
This bit was not supported in the original 80386,
but is supported on more recent x86 processors.
You will therefore have to refer to
Volume 3
of the current Intel manuals.
Make sure you design the kernel to use this optimization
only on processors that support it! Note: If you compiled bochs yourself, be sure that the appropriate configuration options were specified. By default bochs does not support some extended page table features. |
|
Challenge!
Since our JOS kernel's memory management system
only allocates and frees memory on page granularity,
we do not have anything comparable
to a general-purpose malloc/free facility
that we can use within the kernel.
This could be a problem if we want to support
certain types of I/O devices
that require physically contiguous buffers
larger than 4KB in size,
or if we want user-level environments,
and not just the kernel,
to be able to allocate and map 4MB superpages
for maximum processor efficiency.
(See the earlier challenge problem about PTE_PS.) Note: If you compiled bochs yourself, be sure that the appropriate configuration options were specified. By default bochs does not support some extended page table features. Generalize the kernel's memory allocation system to support pages of a variety of power-of-two allocation unit sizes from 4KB up to some reasonable maximum of your choice. Be sure you have some way to divide larger allocation units into smaller ones on demand, and to coalesce multiple small allocation units back into larger units when possible. Think about the issues that might arise in such a system. |
Challenge!
Extend the JOS kernel monitor with commands to
allocate and free pages explicitly,
and display whether or not any given page of physical memory
is currently allocated.
For example:
K> alloc_page 0x13000 K> page_status 0x13000 allocated K> free_page 0x13000 K> page_status 0x13000 freeThink of other commands or extensions to these commands that may be useful for debugging, and add them.
|
This completes the lab.
| Turn-in procedure |