text: #include "param.h"
  #include "types.h"
  #include "defs.h"
  #include "x86.h"
  #include "mmu.h"
  #include "proc.h"
  #include "elf.h"
  
  extern char data[];  // defined in data.S
  
  static pde_t *kpgdir;  // for use in scheduler()
  
  // Allocate one page table for the machine for the kernel address
  // space for scheduler processes.
  void
  kvmalloc(void)
  {
    kpgdir = setupkvm();
  }
  
  // Set up CPU's kernel segment descriptors.
  // Run once at boot time on each CPU.
  void
  seginit(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.
    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);
  
    // 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;
  }
  
  // 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.
  static pte_t *
  walkpgdir(pde_t *pgdir, const void *va, int create)
  {
    pde_t *pde;
    pte_t *pgtab;
  
    pde = &pgdir[PDX(va)];
    if(*pde & PTE_P){
      pgtab = (pte_t*)PTE_ADDR(*pde);
    } else {
      if(!create || (pgtab = (pte_t*)kalloc()) == 0)
        return 0;
      // 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(pgtab) | 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.
  static int
  mappages(pde_t *pgdir, void *la, uint size, uint pa, int perm)
  {
    char *a, *last;
    pte_t *pte;
    
    a = PGROUNDDOWN(la);
    last = PGROUNDDOWN(la + size - 1); 
    // PGROUNDDOWN: get the page aligned address corresponding to la
    // if la = 0x3, then PGROUNDOWN returns 0x0, 
    // which is the nearest 4096-byte page's starting addr.
  
    for(;;){
      pte = walkpgdir(pgdir, a, 1); 
          // walkpgdir goes into the directory, index into it,
          // then find the inner page table, index into it,
          // return PTE
  
      if(pte == 0)
        return -1;
      if(*pte & PTE_P)
        panic("remap");
  
      // Filling in PTE with physical address pa 
      // pa bitwiseOR permision bit bitwiseOR present bit:
      *pte = pa | perm | PTE_P;
      
      if(a == last)
        break;
      a += PGSIZE;
      pa += PGSIZE;
    }
    return 0;
  }
  
  // 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..end          : mapped direct (for the kernel's text and data)
  //   end..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
  // 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).
  static struct kmap {
    void *p;
    void *e;
    int perm;
  } kmap[] = {
    {(void*)USERTOP,    (void*)0x100000, PTE_W},  // I/O space
    {(void*)0x100000,   data,            0    },  // kernel text, rodata
    {data,              (void*)PHYSTOP,  PTE_W},  // kernel data, memory
    {(void*)0xFE000000, 0,               PTE_W},  // device mappings
  };
  
  // Set up kernel part of a page table.
  pde_t*
  setupkvm(void)
  {
    pde_t *pgdir;
    struct kmap *k;
  
    if((pgdir = (pde_t*)kalloc()) == 0)
      return 0;
    memset(pgdir, 0, PGSIZE);
    k = kmap;
    for(k = kmap; k < &kmap[NELEM(kmap)]; k++)
      if(mappages(pgdir, k->p, k->e - k->p, (uint)k->p, k->perm) < 0)
        return 0;
  
    return pgdir;
  }
  
  // Turn on paging.
  void
  vmenable(void)
  {
    uint cr0;
  
    switchkvm(); // load kpgdir into cr3
    cr0 = rcr0();
    cr0 |= CR0_PG;
    lcr0(cr0);
  }
  
  // Switch h/w page table register to the kernel-only page table,
  // for when no process is running.
  void
  switchkvm(void)
  {
    lcr3(PADDR(kpgdir));   // switch to the kernel page table
  }
  
  // Switch TSS and h/w page table to correspond to process p.
  void
  switchuvm(struct proc *p)
  {
    pushcli();
    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");
    lcr3(PADDR(p->pgdir));  // switch to new address space
    popcli();
  }
  
  // Load the initcode into address 0 of pgdir.
  // sz must be less than a page.
  void
  inituvm(pde_t *pgdir, char *init, uint sz)
  {
    char *mem;
    
    if(sz >= PGSIZE)
      panic("inituvm: more than a page");
    mem = kalloc();
    memset(mem, 0, PGSIZE);
    mappages(pgdir, 0, PGSIZE, PADDR(mem), PTE_W|PTE_U);
    memmove(mem, init, sz);
  }
  
  // Load a program segment into pgdir.  addr must be page-aligned
  // and the pages from addr to addr+sz must already be mapped.
  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");
    for(i = 0; i < sz; i += PGSIZE){
      if((pte = walkpgdir(pgdir, addr+i, 0)) == 0)
        panic("loaduvm: address should exist");
      pa = PTE_ADDR(*pte);
      if(sz - i < PGSIZE)
        n = sz - i;
      else
        n = PGSIZE;
      if(readi(ip, (char*)pa, offset+i, n) != n)
        return -1;
    }
    return 0;
  }
  
  // Allocate page tables and physical memory to grow process from oldsz to
  // newsz, which need not be page aligned.  Returns new size or 0 on error.
  int
  allocuvm(pde_t *pgdir, uint oldsz, uint newsz)
  {
    char *mem;
    uint a;
  
    if(newsz > USERTOP)
      return 0;
    if(newsz < oldsz)
      return oldsz;
  
    a = PGROUNDUP(oldsz);
    for(; a < newsz; a += PGSIZE){
      mem = kalloc();
      if(mem == 0){
        cprintf("allocuvm out of memory\n");
        deallocuvm(pgdir, newsz, oldsz);
        return 0;
      }
      memset(mem, 0, PGSIZE);
      mappages(pgdir, (char*)a, PGSIZE, PADDR(mem), PTE_W|PTE_U);
    }
    return newsz;
  }
  
  // Deallocate user pages to bring the process size from oldsz to
  // newsz.  oldsz and newsz need not be page-aligned, nor does newsz
  // need to be less than oldsz.  oldsz can be larger than the actual
  // process size.  Returns the new process size.
  int
  deallocuvm(pde_t *pgdir, uint oldsz, uint newsz)
  {
    pte_t *pte;
    uint a, pa;
  
    if(newsz >= oldsz)
      return oldsz;
  
    a = PGROUNDUP(newsz);
    for(; a  < oldsz; a += PGSIZE){
      pte = walkpgdir(pgdir, (char*)a, 0);
      if(pte && (*pte & PTE_P) != 0){
        pa = PTE_ADDR(*pte);
        if(pa == 0)
          panic("kfree");
        kfree((char*)pa);
        *pte = 0;
      }
    }
    return newsz;
  }
  
  // Free a page table and all the physical memory pages
  // in the user part.
  void
  freevm(pde_t *pgdir)
  {
    uint i;
  
    if(pgdir == 0)
      panic("freevm: no pgdir");
    deallocuvm(pgdir, USERTOP, 0);
    for(i = 0; i < NPDENTRIES; i++){
      if(pgdir[i] & PTE_P)
        kfree((char*)PTE_ADDR(pgdir[i]));
    }
    kfree((char*)pgdir);
  }
  
  // Given a parent process's page table, create a copy
  // of it for a child.
  pde_t*
  copyuvm(pde_t *pgdir, uint sz)
  {
    pde_t *d;
    pte_t *pte;
    uint pa, i;
    char *mem;
  
    if((d = setupkvm()) == 0)
      return 0;
    for(i = 0; i < sz; i += PGSIZE){
      if((pte = walkpgdir(pgdir, (void*)i, 0)) == 0)
        panic("copyuvm: pte should exist");
      if(!(*pte & PTE_P))
        panic("copyuvm: page not present");
  
      pa = PTE_ADDR(*pte);
  
      if((mem = kalloc()) == 0)
        goto bad;
      memmove(mem, (char*)pa, PGSIZE);
  
      // Map pages from parent's to child's address space:
      // Check if the original pte is protected from writing:
      if ((*pte & PTE_W) == 0)
      {
        // Keep writable permission not granted: 
        if(mappages(d, (void*)i, PGSIZE, PADDR(mem), PTE_U) < 0)
          goto bad; 
      }
      else if(mappages(d, (void*)i, PGSIZE, PADDR(mem), PTE_W|PTE_U) < 0)
        goto bad;
  
    }
    return d;
  
  bad:
    freevm(d);
    return 0;
  }
  
  // Map user virtual address to kernel physical address.
  char*
  uva2ka(pde_t *pgdir, char *uva)
  {
    pte_t *pte;
  
    pte = walkpgdir(pgdir, uva, 0);
    if((*pte & PTE_P) == 0)
      return 0;
    if((*pte & PTE_U) == 0)
      return 0;
    return (char*)PTE_ADDR(*pte);
  }
  
  // Copy len bytes from p to user address va in page table pgdir.
  // Most useful when pgdir is not the current page table.
  // uva2ka ensures this only works for PTE_U pages.
  int
  copyout(pde_t *pgdir, uint va, void *p, uint len)
  {
    char *buf, *pa0;
    uint n, va0;
    
    buf = (char*)p;
    while(len > 0){
      va0 = (uint)PGROUNDDOWN(va);
      pa0 = uva2ka(pgdir, (char*)va0);
      if(pa0 == 0)
        return -1;
      n = PGSIZE - (va - va0);
      if(n > len)
        n = len;
      memmove(pa0 + (va - va0), buf, n);
      len -= n;
      buf += n;
      va = va0 + PGSIZE;
    }
    return 0;
  }
  