xv6/vm.c

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#include "param.h"
#include "types.h"
#include "defs.h"
#include "x86.h"
#include "mmu.h"
#include "proc.h"
#include "elf.h"
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// The mappings from logical to linear are one to one (i.e.,
// segmentation doesn't do anything).
// There is one page table per process, plus one that's used
// when a CPU is not running any process (kpgdir).
// A user process uses the same page table as the kernel; the
// page protection bits prevent it from using anything other
// than its memory.
//
// setupkvm() and exec() set up every page table like this:
// 0..640K : user memory (text, data, stack, heap)
// 640K..1M : mapped direct (for IO space)
// 1M..kernend : mapped direct (for the kernel's text and data)
// kernend..PHYSTOP : mapped direct (kernel heap and user pages)
// 0xfe000000..0 : mapped direct (devices such as ioapic)
//
// The kernel allocates memory for its heap and for user memory
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// between kernend and the end of physical memory (PHYSTOP).
// The virtual address space of each user program includes the kernel
// (which is inaccessible in user mode). The user program addresses
// range from 0 till 640KB (USERTOP), which where the I/O hole starts
// (both in physical memory and in the kernel's virtual address
// space).
#define USERTOP 0xA0000
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static uint kerntext; // Linker starts kernel at 1MB
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static uint kerntsz;
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static uint kerndata;
static uint kerndsz;
static uint kernend;
static uint freesz;
static pde_t *kpgdir; // for use in scheduler()
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// return the address of the PTE in page table pgdir
// that corresponds to linear address va. if create!=0,
// create any required page table pages.
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static pte_t *
walkpgdir(pde_t *pgdir, const void *va, int create)
{
uint r;
pde_t *pde;
pte_t *pgtab;
pde = &pgdir[PDX(va)];
if (*pde & PTE_P) {
pgtab = (pte_t*) PTE_ADDR(*pde);
} else if (!create || !(r = (uint) kalloc()))
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return 0;
else {
pgtab = (pte_t*) r;
// Make sure all those PTE_P bits are zero.
memset(pgtab, 0, PGSIZE);
// The permissions here are overly generous, but they can
// be further restricted by the permissions in the page table
// entries, if necessary.
*pde = PADDR(r) | PTE_P | PTE_W | PTE_U;
}
return &pgtab[PTX(va)];
}
// create PTEs for linear addresses starting at la that refer to
// physical addresses starting at pa. la and size might not
// be page-aligned.
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static int
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mappages(pde_t *pgdir, void *la, uint size, uint pa, int perm)
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{
char *first = PGROUNDDOWN(la);
char *last = PGROUNDDOWN(la + size - 1);
char *a = first;
while(1){
pte_t *pte = walkpgdir(pgdir, a, 1);
if(pte == 0)
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return 0;
if(*pte & PTE_P)
panic("remap");
*pte = pa | perm | PTE_P;
if(a == last)
break;
a += PGSIZE;
pa += PGSIZE;
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}
return 1;
}
// Set up CPU's kernel segment descriptors.
// Run once at boot time on each CPU.
void
ksegment(void)
{
struct cpu *c;
// Map virtual addresses to linear addresses using identity map.
// Cannot share a CODE descriptor for both kernel and user
// because it would have to have DPL_USR, but the CPU forbids
// an interrupt from CPL=0 to DPL=3.
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c = &cpus[cpunum()];
c->gdt[SEG_KCODE] = SEG(STA_X|STA_R, 0, 0xffffffff, 0);
c->gdt[SEG_KDATA] = SEG(STA_W, 0, 0xffffffff, 0);
c->gdt[SEG_UCODE] = SEG(STA_X|STA_R, 0, 0xffffffff, DPL_USER);
c->gdt[SEG_UDATA] = SEG(STA_W, 0, 0xffffffff, DPL_USER);
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// map cpu, and curproc
c->gdt[SEG_KCPU] = SEG(STA_W, &c->cpu, 8, 0);
lgdt(c->gdt, sizeof(c->gdt));
loadgs(SEG_KCPU << 3);
// Initialize cpu-local storage.
cpu = c;
proc = 0;
}
// Switch h/w page table and TSS registers to point to process p.
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void
switchuvm(struct proc *p)
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{
pushcli();
// Setup TSS
cpu->gdt[SEG_TSS] = SEG16(STS_T32A, &cpu->ts, sizeof(cpu->ts)-1, 0);
cpu->gdt[SEG_TSS].s = 0;
cpu->ts.ss0 = SEG_KDATA << 3;
cpu->ts.esp0 = (uint)proc->kstack + KSTACKSIZE;
ltr(SEG_TSS << 3);
if (p->pgdir == 0)
panic("switchuvm: no pgdir\n");
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lcr3(PADDR(p->pgdir)); // switch to new address space
popcli();
}
// Switch h/w page table register to the kernel-only page table, for when
// no process is running.
void
switchkvm()
{
lcr3(PADDR(kpgdir)); // Switch to the kernel page table
}
// Set up kernel part of a page table.
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pde_t*
setupkvm(void)
{
pde_t *pgdir;
// Allocate page directory
if (!(pgdir = (pde_t *) kalloc()))
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return 0;
memset(pgdir, 0, PGSIZE);
// Map IO space from 640K to 1Mbyte
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if (!mappages(pgdir, (void *)USERTOP, 0x60000, USERTOP, PTE_W))
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return 0;
// Map kernel text read-only
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if (!mappages(pgdir, (void *) kerntext, kerntsz, kerntext, 0))
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return 0;
// Map kernel data read/write
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if (!mappages(pgdir, (void *) kerndata, kerndsz, kerndata, PTE_W))
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return 0;
// Map dynamically-allocated memory read/write (kernel stacks, user mem)
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if (!mappages(pgdir, (void *) kernend, freesz, PADDR(kernend), PTE_W))
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return 0;
// Map devices such as ioapic, lapic, ...
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if (!mappages(pgdir, (void *)0xFE000000, 0x2000000, 0xFE000000, PTE_W))
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return 0;
return pgdir;
}
// return the physical address that a given user address
// maps to. the result is also a kernel logical address,
// since the kernel maps the physical memory allocated to user
// processes directly.
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char*
uva2ka(pde_t *pgdir, char *uva)
{
pte_t *pte = walkpgdir(pgdir, uva, 0);
if (pte == 0) return 0;
uint pa = PTE_ADDR(*pte);
return (char *)pa;
}
// allocate sz bytes more memory for a process starting at the
// given user address; allocates physical memory and page
// table entries. addr and sz need not be page-aligned.
// it is a no-op for any parts of the requested memory
// that are already allocated.
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int
allocuvm(pde_t *pgdir, char *addr, uint sz)
{
if (addr + sz > (char*)USERTOP)
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return 0;
char *first = PGROUNDDOWN(addr);
char *last = PGROUNDDOWN(addr + sz - 1);
char *a;
for(a = first; a <= last; a += PGSIZE){
pte_t *pte = walkpgdir(pgdir, a, 0);
if(pte == 0 || (*pte & PTE_P) == 0){
char *mem = kalloc();
if(mem == 0){
// XXX clean up?
return 0;
}
memset(mem, 0, PGSIZE);
mappages(pgdir, a, PGSIZE, PADDR(mem), PTE_W|PTE_U);
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}
}
return 1;
}
// deallocate some of the user pages, in response to sbrk()
// with a negative argument. if addr is not page-aligned,
// then only deallocates starting at the next page boundary.
int
deallocuvm(pde_t *pgdir, char *addr, uint sz)
{
if (addr + sz > (char*)USERTOP)
return 0;
char *first = (char*) PGROUNDUP((uint)addr);
char *last = PGROUNDDOWN(addr + sz - 1);
char *a;
for(a = first; a <= last; a += PGSIZE){
pte_t *pte = walkpgdir(pgdir, a, 0);
if(pte && (*pte & PTE_P) != 0){
uint pa = PTE_ADDR(*pte);
if(pa == 0)
panic("deallocuvm");
kfree((void *) pa);
*pte = 0;
}
}
return 1;
}
// free a page table and all the physical memory pages
// in the user part.
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void
freevm(pde_t *pgdir)
{
uint i, j, da;
if (!pgdir)
panic("freevm: no pgdir\n");
for (i = 0; i < NPDENTRIES; i++) {
da = PTE_ADDR(pgdir[i]);
if (da != 0) {
pte_t *pgtab = (pte_t*) da;
for (j = 0; j < NPTENTRIES; j++) {
if (pgtab[j] != 0) {
uint pa = PTE_ADDR(pgtab[j]);
uint va = PGADDR(i, j, 0);
if (va < USERTOP) // user memory
kfree((void *) pa);
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pgtab[j] = 0;
}
}
kfree((void *) da);
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pgdir[i] = 0;
}
}
kfree((void *) pgdir);
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}
int
loaduvm(pde_t *pgdir, char *addr, struct inode *ip, uint offset, uint sz)
{
uint i, pa, n;
pte_t *pte;
if ((uint)addr % PGSIZE != 0)
panic("loaduvm: addr must be page aligned\n");
for (i = 0; i < sz; i += PGSIZE) {
if (!(pte = walkpgdir(pgdir, addr+i, 0)))
panic("loaduvm: address should exist\n");
pa = PTE_ADDR(*pte);
if (sz - i < PGSIZE) n = sz - i;
else n = PGSIZE;
if(readi(ip, (char *)pa, offset+i, n) != n)
return 0;
}
return 1;
}
void
inituvm(pde_t *pgdir, char *addr, char *init, uint sz)
{
uint i, pa, n, off;
pte_t *pte;
for (i = 0; i < sz; i += PGSIZE) {
if (!(pte = walkpgdir(pgdir, (void *)(i+addr), 0)))
panic("inituvm: pte should exist\n");
off = (i+(uint)addr) % PGSIZE;
pa = PTE_ADDR(*pte);
if (sz - i < PGSIZE) n = sz - i;
else n = PGSIZE;
memmove((char *)pa+off, init+i, n);
}
}
// given a parent process's page table, create a copy
// of it for a child.
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pde_t*
copyuvm(pde_t *pgdir, uint sz)
{
pde_t *d = setupkvm();
pte_t *pte;
uint pa, i;
char *mem;
if (!d) return 0;
for (i = 0; i < sz; i += PGSIZE) {
if (!(pte = walkpgdir(pgdir, (void *)i, 0)))
panic("copyuvm: pte should exist\n");
if(*pte & PTE_P){
pa = PTE_ADDR(*pte);
if (!(mem = kalloc()))
return 0;
memmove(mem, (char *)pa, PGSIZE);
if (!mappages(d, (void *)i, PGSIZE, PADDR(mem), PTE_W|PTE_U))
return 0;
}
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}
return d;
}
// Gather information about physical memory layout.
// Called once during boot.
// Really should find out how much physical memory
// there is rather than assuming PHYSTOP.
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void
pminit(void)
{
extern char end[];
struct proghdr *ph;
struct elfhdr *elf = (struct elfhdr*)0x10000; // scratch space
if (elf->magic != ELF_MAGIC || elf->phnum != 2)
panic("pminit: need a text and data segment\n");
ph = (struct proghdr*)((uchar*)elf + elf->phoff);
kernend = ((uint)end + PGSIZE) & ~(PGSIZE-1);
kerntext = ph[0].va;
kerndata = ph[1].va;
kerntsz = ph[0].memsz;
kerndsz = ph[1].memsz;
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freesz = PHYSTOP - kernend;
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kinit((char *)kernend, freesz);
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}
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// Allocate one page table for the machine for the kernel address
// space for scheduler processes.
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void
kvmalloc(void)
{
kpgdir = setupkvm();
}
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// Turn on paging.
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void
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vmenable(void)
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{
uint cr0;
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switchkvm(); // load kpgdir into cr3
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cr0 = rcr0();
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cr0 |= CR0_PG;
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lcr0(cr0);
}