xv6 内核内存

内核内存初始化过程

在main.c种 kinit()和 kvminit()是初始化物理内存和虚拟内存的主要代码

void
main()
{
    consoleinit();
    printfinit();
    printf("\n");
    printf("xv6 kernel is booting\n");
    printf("\n");
    kinit();         // 物理内存的初始化
    kvminit();       // 创建页表，并分配一些
    kvminithart();   // 保存页表地址，开始使用虚拟地址
	//.....省略其他初始化过程

  scheduler();        
}

物理地址的范围 0x80000000开始 共128M


// 物理地址的范围 0x80000000开始 共128M
//提供给内核使用
#define KERNBASE 0x80000000L
#define PHYSTOP (KERNBASE + 128*1024*1024)

kinit：初始化物理内存

初始化物理内存，构成页的链表，每页4096字节，方便管理分配和回收

#define PGSIZE 4096 

void
kinit()
{
  initlock(&kmem.lock, "kmem");//加锁
  freerange(end, (void*)PHYSTOP);
}
void
freerange(void *pa_start, void *pa_end)
{
  char *p;
  p = (char*)PGROUNDUP((uint64)pa_start);
  for(; p + PGSIZE <= (char*)pa_end; p += PGSIZE)
    kfree(p);
}

void
kfree(void *pa)
{
  struct run *r;

  if(((uint64)pa % PGSIZE) != 0 || (char*)pa < end || (uint64)pa >= PHYSTOP)
    panic("kfree");

  // Fill with junk to catch dangling refs.
  memset(pa, 1, PGSIZE);

  r = (struct run*)pa;

  acquire(&kmem.lock);
  r->next = kmem.freelist;
  kmem.freelist = r;
  release(&kmem.lock);
}

kvminit：初始化页表

初始化页表；

初始化一些固定配置的虚拟地址和页表的映射；

pagetable_t
kvmmake(void)
{
  pagetable_t kpgtbl;
  //分配一个物理页，并返回物理页的地址，用来存放页表
  kpgtbl = (pagetable_t) kalloc();
  memset(kpgtbl, 0, PGSIZE);

  //分配一个物理页 uart registers
  kvmmap(kpgtbl, UART0, UART0, PGSIZE, PTE_R | PTE_W);

  // virtio mmio disk interface
  kvmmap(kpgtbl, VIRTIO0, VIRTIO0, PGSIZE, PTE_R | PTE_W);

  // PLIC
  kvmmap(kpgtbl, PLIC, PLIC, 0x400000, PTE_R | PTE_W);

  // map kernel text executable and read-only.
  kvmmap(kpgtbl, KERNBASE, KERNBASE, (uint64)etext-KERNBASE, PTE_R | PTE_X);

  // map kernel data and the physical RAM we'll make use of.
  kvmmap(kpgtbl, (uint64)etext, (uint64)etext, PHYSTOP-(uint64)etext, PTE_R | PTE_W);

  // map the trampoline for trap entry/exit to
  // the highest virtual address in the kernel.
  kvmmap(kpgtbl, TRAMPOLINE, (uint64)trampoline, PGSIZE, PTE_R | PTE_X);

  // allocate and map a kernel stack for each process.
  proc_mapstacks(kpgtbl);
  
  return kpgtbl;
}

因为一个页表项是代表一页的，所以需要通过虚拟地址的起始位置和size来判断需要几个页表项； 页表项的值就需要通过物理地址来构建，物理页号和权限（*pte = PA2PTE(pa) | perm | PTE_V）；

void
kvmmap(pagetable_t kpgtbl, uint64 va, uint64 pa, uint64 sz, int perm)
{
  if(mappages(kpgtbl, va, sz, pa, perm) != 0)
    panic("kvmmap");
}

// Create PTEs for virtual addresses starting at va that refer to
// physical addresses starting at pa. va and size might not
// be page-aligned. Returns 0 on success, -1 if walk() couldn't
// allocate a needed page-table page.
int
mappages(pagetable_t pagetable, uint64 va, uint64 size, uint64 pa, int perm)
{
  uint64 a, last;
  pte_t *pte;

  if(size == 0)
    panic("mappages: size");
  
  a = PGROUNDDOWN(va);
  last = PGROUNDDOWN(va + size - 1);
  for(;;){
    if((pte = walk(pagetable, a, 1)) == 0)
      return -1;
    if(*pte & PTE_V)
      panic("mappages: remap");
    *pte = PA2PTE(pa) | perm | PTE_V;
    if(a == last)
      break;
    a += PGSIZE;
    pa += PGSIZE;
  }
  return 0;
}