MIT_6.828_lab2_exercise5_讲解
exercise5要求补充mem_init()
完全按照提示来就行了。
// Set up a two-level page table:
// kern_pgdir is its linear (virtual) address of the root
//
// This function only sets up the kernel part of the address space
// (ie. addresses >= UTOP). The user part of the address space
// will be set up later.
//
// From UTOP to ULIM, the user is allowed to read but not write.
// Above ULIM the user cannot read or write.
void
mem_init(void)
{
uint32_t cr0;
size_t n;
// Find out how much memory the machine has (npages & npages_basemem).
i386_detect_memory();
// Remove this line when you're ready to test this function.
//panic("mem_init: This function is not finished\n");
//
// create initial page directory.
kern_pgdir = (pde_t *) boot_alloc(PGSIZE);
memset(kern_pgdir, 0, PGSIZE);
//
// Recursively insert PD in itself as a page table, to form
// a virtual page table at virtual address UVPT.
// (For now, you don't have understand the greater purpose of the
// following line.)
// Permissions: kernel R, user R
kern_pgdir[PDX(UVPT)] = PADDR(kern_pgdir) | PTE_U | PTE_P;
//这句后面的两个按位或，是设置权限user、present。
//
// Allocate an array of npages 'struct PageInfo's and store it in 'pages'.
// The kernel uses this array to keep track of physical pages: for
// each physical page, there is a corresponding struct PageInfo in this
// array. 'npages' is the number of physical pages in memory. Use memset
// to initialize all fields of each struct PageInfo to 0.
// Your code goes here:
pages = (struct PageInfo*) boot_alloc(sizeof(struct PageInfo) * npages);
memset(pages, 0, sizeof(struct PageInfo) * npages);
//
// Now that we've allocated the initial kernel data structures, we set
// up the list of free physical pages. Once we've done so, all further
// memory management will go through the page_* functions. In
// particular, we can now map memory using boot_map_region
// or page_insert
page_init();
check_page_free_list(1);
check_page_alloc();
check_page();
//
// Now we set up virtual memory
//
// Map 'pages' read-only by the user at linear address UPAGES
// Permissions:
// - the new image at UPAGES -- kernel R, user R
// (ie. perm = PTE_U | PTE_P)
// - pages itself -- kernel RW, user NONE
// Your code goes here:
boot_map_region(kern_pgdir, UPAGES, sizeof(struct PageInfo) * npages, PADDR(pages), PTE_U);
//
// Use the physical memory that 'bootstack' refers to as the kernel
// stack. The kernel stack grows down from virtual address KSTACKTOP.
// We consider the entire range from [KSTACKTOP-PTSIZE, KSTACKTOP)
// to be the kernel stack, but break this into two pieces:
// * [KSTACKTOP-KSTKSIZE, KSTACKTOP) -- backed by physical memory
// * [KSTACKTOP-PTSIZE, KSTACKTOP-KSTKSIZE) -- not backed; so if
// the kernel overflows its stack, it will fault rather than
// overwrite memory. Known as a "guard page".
// Permissions: kernel RW, user NONE
// Your code goes here:
boot_map_region(kern_pgdir, KSTACKTOP - KSTKSIZE, KSTKSIZE, PADDR(bootstack), PTE_W);
//
// Map all of physical memory at KERNBASE.
// Ie. the VA range [KERNBASE, 2^32) should map to
// the PA range [0, 2^32 - KERNBASE)
// We might not have 2^32 - KERNBASE bytes of physical memory, but
// we just set up the mapping anyway.
// Permissions: kernel RW, user NONE
// Your code goes here:
boot_map_region(kern_pgdir, KERNBASE, 0xffffffff - KERNBASE, 0, PTE_W);
// Check that the initial page directory has been set up correctly.
check_kern_pgdir();
// Switch from the minimal entry page directory to the full kern_pgdir
// page table we just created. Our instruction pointer should be
// somewhere between KERNBASE and KERNBASE+4MB right now, which is
// mapped the same way by both page tables.
//
// If the machine reboots at this point, you've probably set up your
// kern_pgdir wrong.
lcr3(PADDR(kern_pgdir));
check_page_free_list(0);
// entry.S set the really important flags in cr0 (including enabling
// paging). Here we configure the rest of the flags that we care about.
cr0 = rcr0();
cr0 |= CR0_PE|CR0_PG|CR0_AM|CR0_WP|CR0_NE|CR0_MP;
cr0 &= ~(CR0_TS|CR0_EM);
lcr0(cr0);
// Some more checks, only possible after kern_pgdir is installed.
check_page_installed_pgdir();
}
第三句boot_map_region中，注意要使用0xffffffff，而不是1<<32，因为地址是从0开始的，后者多了一位。
下面再附上虚拟内存分布图
/*
* Virtual memory map: Permissions
* kernel/user
*
* 4 Gig --------> +------------------------------+
* | | RW/--
* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* : . :
* : . :
* : . :
* |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| RW/--
* | | RW/--
* | Remapped Physical Memory | RW/--
* | | RW/--
* KERNBASE, ----> +------------------------------+ 0xf0000000 --+
* KSTACKTOP | CPU0's Kernel Stack | RW/-- KSTKSIZE |
* | - - - - - - - - - - - - - - -| |
* | Invalid Memory (*) | --/-- KSTKGAP |
* +------------------------------+ |
* | CPU1's Kernel Stack | RW/-- KSTKSIZE |
* | - - - - - - - - - - - - - - -| PTSIZE
* | Invalid Memory (*) | --/-- KSTKGAP |
* +------------------------------+ |
* : . : |
* : . : |
* MMIOLIM ------> +------------------------------+ 0xefc00000 --+
* | Memory-mapped I/O | RW/-- PTSIZE
* ULIM, MMIOBASE --> +------------------------------+ 0xef800000
* | Cur. Page Table (User R-) | R-/R- PTSIZE
* UVPT ----> +------------------------------+ 0xef400000
* | RO PAGES | R-/R- PTSIZE
* UPAGES ----> +------------------------------+ 0xef000000
* | RO ENVS | R-/R- PTSIZE
* UTOP,UENVS ------> +------------------------------+ 0xeec00000
* UXSTACKTOP -/ | User Exception Stack | RW/RW PGSIZE
* +------------------------------+ 0xeebff000
* | Empty Memory (*) | --/-- PGSIZE
* USTACKTOP ---> +------------------------------+ 0xeebfe000
* | Normal User Stack | RW/RW PGSIZE
* +------------------------------+ 0xeebfd000
* | |
* | |
* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* . .
* . .
* . .
* |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
* | Program Data & Heap |
* UTEXT --------> +------------------------------+ 0x00800000
* PFTEMP -------> | Empty Memory (*) | PTSIZE
* | |
* UTEMP --------> +------------------------------+ 0x00400000 --+
* | Empty Memory (*) | |
* | - - - - - - - - - - - - - - -| |
* | User STAB Data (optional) | PTSIZE
* USTABDATA ----> +------------------------------+ 0x00200000 |
* | Empty Memory (*) | |
* 0 ------------> +------------------------------+ --+
*
* (*) Note: The kernel ensures that "Invalid Memory" is *never* mapped.
* "Empty Memory" is normally unmapped, but user programs may map pages
* there if desired. JOS user programs map pages temporarily at UTEMP.
*/