Spring 2017 :: CSE 506 — Operating Systems

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Lab 2 Introduction

In this lab, you will write the memory management code for your operating system. Memory management has two components.

The first component is a physical memory allocator for the kernel, so that the kernel can allocate memory and later free it. Your allocator will operate in units of 4096 bytes, called pages. Your task will be to maintain data structures that record which physical pages are free and which are allocated, and how many processes are sharing each allocated page. You will also write the routines to allocate and free pages of memory.

The second component of memory management is virtual memory, which maps the virtual addresses used by kernel and user software to addresses in physical memory. The amd64 hardware's memory management unit (MMU) performs the mapping when instructions use memory, consulting a set of page tables. You will modify JOS to set up the MMU's page tables according to a specification we provide.

Getting Started

In this and future labs you will progressively build up your kernel. We will also provide you with some additional source. Follow these instructions to fetch the new source and merge it with your old code. Remember to use lab2 as the name of the new Git branch.

Lab 2 contains the following new source files, which you should browse through:

• inc/memlayout.h
• kern/pmap.c
• kern/pmap.h
• kern/kclock.h
• kern/kclock.c

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 PageInfo 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.

Pay particular attention to memlayout.h and pmap.h, since this lab requires you to use and understand many of the definitions they contain. You may want to reviewinc/mmu.h, too, as it also contains a number of definitions that will be useful for this lab.

Part 1: Physical Page Management

The operating system must keep track of which parts of physical RAM are free and which are currently in use. JOS manages the PC's physical memory with page granularity so that it can use the MMU to map and protect each piece of allocated memory.

JOS is "told" the amount of physical memory it has by the bootloader. JOS's bootloader passes the kernel a multiboot info structure which possibly contains the physical memory map of the system. The memory map may exclude regions of memory that are in use for reasons including IO mappings for devices (e.g., the "memory hole"), space reserved for the BIOS, or physically damaged memory. For more details on how this structure looks and what it contains, refer to the specification. A typical physical memory map for a PC with 10 GB of memory looks like below.

e820 MEMORY MAP
		address: 0x0000000000000000, length: 0x000000000009f400, type: USABLE
		address: 0x000000000009f400, length: 0x0000000000000c00, type: RESERVED
		address: 0x00000000000f0000, length: 0x0000000000010000, type: RESERVED
		address: 0x0000000000100000, length: 0x00000000dfefd000, type: USABLE
		address: 0x00000000dfffd000, length: 0x0000000000003000, type: RESERVED
		address: 0x00000000fffc0000, length: 0x0000000000040000, type: RESERVED
		address: 0x0000000100000000, length: 0x00000001a0000000, type: USABLE

You'll now write the physical page allocator. It keeps track of which pages are free with a linked list of struct PageInfo objects, each corresponding to a physical page. You need to write the physical page allocator before you can write the rest of the virtual memory implementation, because your page table management code will need to allocate physical memory in which to store page tables.

This lab, and all the CSE 506 labs, will require you to do a bit of detective work to figure out exactly what you need to do. This assignment does not describe all the details of the code you'll have to add to JOS. Look for comments in the parts of the JOS source that you have to modify; those comments often contain specifications and hints. You will also need to look at related parts of JOS, at the Intel manuals, and perhaps at your notes from previous Operating Systems courses.

Part 2: Virtual Memory

Before doing anything else, familiarize yourself with the AMD64's long-mode memory management architecture: namely segmentation and page translation.

Virtual, Linear, and Physical Addresses

In AMD64 terminology, a virtual address consists of a segment selector and an offset within the segment. A linear address is what you get after segment translation but before page translation. A physical address is what you finally get after both segment and page translation and what ultimately goes out on the hardware bus to your RAM. Be sure you understand the difference between these three types or "levels" of addresses!

           Selector  +--------------+         +-----------+
          ---------->|              |         |           |
                     | Segmentation |         |  Paging   |
Software             |              |-------->|           |---------->  RAM
            Offset   |  Mechanism   |         | Mechanism |
          ---------->|              |         |           |
                     +--------------+         +-----------+
            Virtual                   Linear                Physical

A C pointer is the "offset" component of the virtual address. In kern/bootstrap.S, we installed a Global Descriptor Table (GDT) that effectively disabled segment translation by setting all segment base addresses to 0 and limits to 0xffffffff. Hence the "selector" has no effect and the linear address always equals the offset of the virtual address. In lab 3, we'll have to interact a little more with segmentation to set up privilege levels, but as for memory translation, we can ignore segmentation throughout the JOS labs and focus solely on page translation.

From code executing on the CPU, once we're in protected/long mode, there's no way to directly use a linear or physical address. All memory references are interpreted as virtual addresses and translated by the MMU, which means all pointers in C are virtual addresses.

The JOS kernel often needs to manipulate addresses as opaque values or as integers, without dereferencing them, for example in the physical memory allocator. Sometimes these are virtual addresses, and sometimes they are physical addresses. To help document the code, the JOS source distinguishes the two cases: the type uintptr_t represents virtual addresses, and physaddr_t represents physical addresses. Both these types are really just synonyms for 64-bit integers (uint64_t), so the compiler won't stop you from assigning one type to another! Since they are integer types (not pointers), the compiler will complain if you try to dereference them.

The JOS kernel can dereference a uintptr_t by first casting it to a pointer type. In contrast, the kernel can't sensibly dereference a physical address, since the MMU translates all memory references. If you cast a physaddr_t to a pointer and dereference it, you may be able to load and store to the resulting address (the hardware will interpret it as a virtual address), but you probably won't get the memory location you intended. To summarize:

In Part 3 of Lab 1 we noted that the kernel's first step is to set up simple segmentation and paging (in kern/bootstrap.S) so that the kernel runs at its link address of 0x8004100000, 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 0x8004100000, but its physical starting address is 0x00100000. The kernel's virtual and linear addresses are same because of the flat segmentation hardware in AMD64, while its linear and physical addresses differ because of the paging hardware (Remember we mapped the upper 256 MB 0xf000000 through 0xffffffff back to 0x0 through 0xfffffff)

However, the JOS kernel sometimes needs to read or modify memory for which it only knows the physical address. For example, adding a mapping to a page table may require allocating physical memory to store a page directory and then initializing that memory. However, the kernel, like any other software, cannot bypass virtual memory translation and thus cannot directly load and store to physical addresses. One reason JOS remaps of all of physical memory starting from physical address 0 at virtual address 0x8004000000 is to help the kernel read and write memory for which it knows just the physical address. In order to translate a physical address into a virtual address that the kernel can actually read and write, the kernel must add 0x8004000000 to the physical address to find its corresponding virtual address in the remapped region. You should use KADDR(pa) to do that addition.

The JOS kernel also sometimes needs to be able to find a physical address given the virtual address of the memory in which a kernel data structure is stored. The kernel addresses its global variables and memory that boot_alloc() allocates, with addresses in the region where the kernel was loaded, starting at 0x8004000000, the very region where we mapped all of physical memory. Thus, to turn a virtual address in this region into a physical address, the kernel can simply subtract 0x8004000000. You should use PADDR(va) to do that subtraction.

Reference counting

In future labs you will often have the same physical page mapped at multiple virtual addresses simultaneously (or in the address spaces of multiple environments). You will keep a count of the number of references to each physical page in the pp_ref field of the struct PageInfo corresponding to the physical page. When this count goes to zero for a physical page, that page can be freed because it is no longer used. In general, this count should equal to the number of times the physical page appears below UTOP in all page tables (the mappings above UTOP are mostly set up at boot time by the kernel and should never be freed, so there's no need to reference count them). We'll also use it to keep track of the number of pointers we keep to the page directory pages and, in turn, of the number of references the page directories have to page table pages.

Be careful when using page_alloc. The page it returns will always have a reference count of 0, so pp_ref should be incremented as soon as you've done something with the returned page (like inserting it into a page table). Sometimes this is handled by other functions (for example, page_insert) and sometimes the function calling page_alloc must do it directly.

Page Table Management

Now you'll write a set of routines to manage page tables: to insert and remove linear-to-physical mappings, and to create page table pages when needed.

pml4e_walk()
pdpe_walk()
pgdir_walk()
boot_map_region()
page_lookup()
page_remove()
page_insert()

Part 3: Kernel Address Space

JOS divides the processor's 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.

You'll find it helpful to refer to the JOS memory layout diagram in inc/memlayout.h both for this part and for later labs.

Permissions and Fault Isolation

Since kernel and user memory are both present in each environment's address space, we will have to use permission bits in our amd64 page tables to allow user code access only to the user part of the address space. Otherwise bugs in user code might overwrite kernel data, causing a crash or more subtle malfunction; user code might also be able to steal other environments' private data.

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.

Initializing the Kernel Address Space

Now you'll set up the address space above UTOP: the kernel part of the address space. inc/memlayout.h shows the layout you should use. You'll use the functions you just wrote to set up the appropriate linear to physical mappings.

Address Space Layout Alternatives

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 user-level processes unrestricted use of the entire virtual address space - while still fully protecting the kernel from these processes and protecting different processes from each other!

This completes the lab. Type make handin in the lab directory. If submission fails, double check that you have committed all of your changes, and read any error messages carefully before emailing the course staff for help.