Lab5: Page Bookkeeping

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Translation Function

Kernel Virtual Address, Physical Address, Page Frame Number 之間的轉換函式

include/mm.h

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#ifndef _MM_H
#define _MM_H

#define KERNEL_VIRT_BASE 0xFFFF000000000000
#define PAGE_TABLE_SIZE 4096
#define PAGE_SHIFT 12
#define PAGE_SIZE (1UL << PAGE_SHIFT)
#define PAGE_MASK (~(PAGE_SIZE - 1))
#define PAGE_NUM (0x40000000 / PAGE_SIZE)

#ifndef __ASSEMBLER__

#include <include/types.h>

#define KVA_TO_PA(addr) ((uint64_t) addr << 16 >> 16)
#define PA_TO_KVA(addr) ((uint64_t) addr | KERNEL_VIRT_BASE)
#define PA_TO_PFN(addr) ((uint64_t) addr >> PAGE_SHIFT)
#define PFN_TO_PA(idx) ((uint64_t) idx << PAGE_SHIFT)

#endif
#endif

Bookkeeping

include/mm.h

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enum page_flag { PAGE_USED = 1 << 0 };

typedef struct {
    uint32_t flag;
    void *physical;
} page_t;

extern page_t page[PAGE_NUM];

kernel/mm.c

kernel image 所佔據的 page、kernel stack 所佔據的 page (0x8000 前)必須標示成使用中

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page_t page[PAGE_NUM];

bool is_set(uint32_t target, uint32_t val)
{
    return !!(target & val);
}

void set(uint32_t *target, uint32_t val)
{
    *target |= val;
}

void unset(uint32_t *target, uint32_t val)
{
    *target &= ~val;
}

void page_init()
{
    extern char _kernel_end;
    int i = 0;
    for (; i < PA_TO_PFN(KVA_TO_PA(&_kernel_end)); ++i) {
        // kernel stack + kernel image
        page[i].physical = (void *) ((physaddr_t) i << PAGE_SHIFT);
        page[i].flag = 0;
        set(&page[i].flag, PAGE_USED);
    }
    for (; i < PA_TO_PFN(KVA_TO_PA(MMIO_BASE)); ++i) {
        page[i].flag = 0;
    }
    for (; i < PAGE_NUM; ++i) {
        // MMIO
        page[i].physical = (void *) ((physaddr_t) i << PAGE_SHIFT);
        page[i].flag = 0;
        set(&page[i].flag, PAGE_USED);
    }
}

kernel/start.S

稍微試算page_t page[PAGE_NUM]的佔用空間

  • PAGE_NUM=1G/4K
  • sizeof(page_t)=8 Bytes
  • sizeof(page[PAGE_NUM])=16 MB

發現已經遠超過 adr 的定址範圍：+-1MB，需要將 .bss 初始化程式更改一下

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    // clear bss
-   adr     x0, __bss_start
-   adr     x1, __bss_end
+   ldr     x0, =__bss_start
+   ldr     x1, =__bss_end
		sub     x1, x1, x0
		bl      memzero

+   bl      page_init
    bl      main

page_alloc, page_free

kernel/mm.h

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page_t *get_free_page();
void *page_alloc_kernel();
void *page_alloc_user();
void page_free(void *virt_addr);

kernel/mm.c

page_alloc_kernel 和 page_alloc_user 取得閒置的 page 並返回 page 的 virtual address 而非 physical address，原因在於一開始我們已啟用 virtual memory。

一旦分配 page，會在 task_struct 紀錄。page_alloc_kernel 和 page_alloc_user 的差別僅紀錄該 page 是分配給 kernel space 或是 user space，但這部份尚未實作，因此兩個函式目前一樣。

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page_t *get_free_page()
{
    extern char _kernel_end;
    int pfn_start = PA_TO_PFN(KVA_TO_PA(&_kernel_end)),
        pfn_end = PA_TO_PFN(KVA_TO_PA(MMIO_BASE));

    // find an available page
    for (; pfn_start < pfn_end && is_set(page[pfn_start].flag, PAGE_USED);
         ++pfn_start)
        ;
    if (pfn_start == pfn_end)
        return NULL;

    // initialize the page
    physaddr_t phy_addr = (physaddr_t) pfn_start << PAGE_SHIFT;
    uintptr_t virt_addr = phy_addr | KERNEL_VIRT_BASE;
    memset((void *) virt_addr, 0, PAGE_SIZE);

    // save to page struct
    page[pfn_start].physical = (void *) phy_addr;
    set(&page[pfn_start].flag, PAGE_USED);

    // return pointer to page struct
    return &page[pfn_start];
}

// get a page for kernel space
void *page_alloc_kernel()
{
    page_t *page_ptr = get_free_page();
    if (!page_ptr)
        return NULL;

    physaddr_t phy_addr = (physaddr_t) page_ptr->physical;
    uintptr_t virt_addr = phy_addr | KERNEL_VIRT_BASE;

    return (void *) virt_addr;
}

// get a page for user space
void *page_alloc_user()
{
    page_t *page_ptr = get_free_page();
    if (!page_ptr)
        return NULL;

    physaddr_t phy_addr = (physaddr_t) page_ptr->physical;
    uintptr_t virt_addr = phy_addr | KERNEL_VIRT_BASE;

    return (void *) virt_addr;
}

void page_free(void *virt_addr)
{
    int pfn = PA_TO_PFN(KVA_TO_PA((uintptr_t) virt_addr));
    unset(&page[pfn].flag, PAGE_USED);
    page[pfn].physical = NULL;
}