matos/core/paging.c

#include "klibc.h"
#include "errno.h"
#include "mem.h"
#include "paging.h"
#include "stdarg.h"
#include "vga.h"

// In a Vaddr, 10 first bit (MSB) are the index in the Page Directory. A Page Directory Entry point to a Page Table.
// The 10 next bits are then an index in this Page Table. A Page Table Entry then point to a physical address at which is added the remaining 12 bits.
// So they are 1024 entry in the PD, each of them pointing to a PT of 1024 entry. Each PTE pointing to 4K page.
// First address (up to page_desc from mem.c) are mapped such as Paddr == Vaddr.
// To make PD always accessible a (x86?) trick is used : The mirroring. A given entry N in the PD point to the PD (this is possible because PDE very looks like PTE in x86).
// So N << (10 + 12 = 4Mo) point to the Paddr of PD. Then, accessing N * 4Mo + I * 4Ko is accessing the PT of the Ieme entry in the PD (as MMU take the PD pointed by the PDE number N like a PT).
// More particularly, accessing N * 4Mo + N * 4ko is accessing the PD.
//
// PD is at Vaddr N * 4Mo and take 4ko. Each PT are allocated dynamically.
// Just make sure that N have not been used by identity mapping

#define PT_SHIFT 12
#define PTE_MASK 0x3ff //10bits
#define PD_SHIFT 22
#define PD_MIRROR_PAGE_IDX 1023U

struct pde {
	uint32_t present : 1;
	uint32_t write : 1;         // 0 read - 1 RW
	uint32_t user : 1;          // 0 supervisor - 1 user
	uint32_t write_through : 1; // 0 write-back - 1 write_through
	uint32_t cache_disable : 1;
	uint32_t access : 1; // have been accessed
	uint32_t zero : 1;   // Not used
	uint32_t size : 1;   // 0 for 4Kb 1 for 4Mb
	uint32_t ignored : 1;
	uint32_t available : 3;
	uint32_t pt_addr : 20;
} __attribute__((packed));

struct pte {
	uint32_t present : 1;
	uint32_t write : 1;         // 0 read - 1 RW
	uint32_t user : 1;          // 0 supervisor - 1 user
	uint32_t write_through : 1; // 0 write-back - 1 write_through
	uint32_t cache_disable : 1;
	uint32_t access : 1; // have been accessed
	uint32_t dirty : 1;  // if set, indicates that page has been written to. This flag is
			     // not updated by the CPU, and once set will not unset itself.
	uint32_t zero : 1;   // if PAT is supported, shall indicate the memory type. Otherwise,
			     // it must be 0.
	uint32_t global : 1; // if set, prevents the TLB from updating the address in its cache
			     // if CR3 is reset. Note, that the page global enable bit in CR4
			     // must be set to enable this feature.
	uint32_t available : 3;
	uint32_t paddr : 20;
} __attribute__((packed));

struct pdbr {
	uint32_t zero1 : 3;          // reserved
	uint32_t write_through : 1;  // 0 write-back - 1 write-through
	uint32_t cache_disabled : 1; // 1=cache disabled
	uint32_t zero2 : 7;          // reserved
	uint32_t pd_paddr : 20;
} __attribute__((packed));


// invalidate the TLB entry for the page located at the given virtual address
static inline void __native_flush_tlb_single(unsigned long addr)
{
	asm volatile("invlpg (%0)" ::"r"(addr) : "memory");
}

int pagingSetup(paddr_t upperKernelAddr)
{
	struct pdbr cr3;

	// x86 got 1024 of pde for 4Byte each: 4ko !
	struct pde *pd = (struct pde *)allocPhyPage();

	memset(pd, 0, PAGE_SIZE);
	memset(&cr3, 0x0, sizeof(struct pdbr));

	cr3.pd_paddr = ((paddr_t)pd) >> 12;

	// MMU not enabled for the moment. No need to use mirroring
	// Identity mapping up to upperKernelAddr
	for (paddr_t i = 0; i < upperKernelAddr; i += PAGE_SIZE) {
		uint pdEntry = i >> (PD_SHIFT);
		uint ptEntry = (i >> PT_SHIFT ) & PTE_MASK;
		struct pte *pt;
		if (pd[pdEntry].present){
			pt = (struct pte *)(pd[pdEntry].pt_addr << PT_SHIFT);
			refPhyPage((paddr_t)pt);
		} else {
			pt = (struct pte *)allocPhyPage();

			memset(pt, 0, PAGE_SIZE);
			pd[pdEntry].present = 1;
			pd[pdEntry].write = 1;
			pd[pdEntry].pt_addr =  ((paddr_t)pt >> PT_SHIFT);
		}

		pt[ptEntry].present = 1;
		pt[ptEntry].write = 1; //TODO set Kernel code as RO
		pt[ptEntry].paddr = i >> PAGE_SHIFT;
	}

	// Setup mirroring
	pd[PD_MIRROR_PAGE_IDX].present = 1;
	pd[PD_MIRROR_PAGE_IDX].write = 1;
	pd[PD_MIRROR_PAGE_IDX].pt_addr = ((paddr_t)pd >> PT_SHIFT);


	// Loading of the PDBR in the MMU:
	asm volatile ("movl %0,%%cr3\n\t"
			"movl %%cr0,%%eax\n\t"
			"orl $0x80010000, %%eax\n\t" /* bit 31 | bit 16 */
			"movl %%eax,%%cr0\n\t"
			"jmp 1f\n\t"
			"1:\n\t"
			"movl $2f, %%eax\n\t"
			"jmp *%%eax\n\t"
			"2:\n\t" ::"r"(cr3):"memory","eax");
	return 0;
}

int pageMap(vaddr_t vaddr, paddr_t paddr, int flags)
{
	uint pdEntry = vaddr >> (PD_SHIFT);
	uint ptEntry = (vaddr >> PT_SHIFT) & PTE_MASK;

	// Thank to mirroring, we can access the PD
	struct pde *pd =
	    (struct pde *)((PD_MIRROR_PAGE_IDX << 22) + (PD_MIRROR_PAGE_IDX << 12));

	struct pte *pt = (struct pte *)((PD_MIRROR_PAGE_IDX << 22) + (pdEntry << 12));

	if (!pd[pdEntry].present) {
		paddr_t ptPhy = allocPhyPage();
		if (ptPhy == (vaddr_t)NULL)
			return ENOMEM;

		pd[pdEntry].user = (flags & PAGING_MEM_USER) ? 1:0;
		pd[pdEntry].present = 1;
		pd[pdEntry].write = 1;
		pd[pdEntry].pt_addr = (ptPhy >> PT_SHIFT);

		__native_flush_tlb_single((vaddr_t)pt);
		memset((void *)pt, 0, PAGE_SIZE);
	}

	// Already mapped ? Remove old mapping
	if (pt[ptEntry].present) {
		unrefPhyPage(pt[ptEntry].paddr << PAGE_SHIFT);
	} // PTE not already used ? We will use it ! So increase the PT ref count
	else {
		refPhyPage(pd[pdEntry].pt_addr << PAGE_SHIFT);
	}

	pt[ptEntry].user    = (flags & PAGING_MEM_USER) ? 1 : 0;
	pt[ptEntry].present = 1;
	pt[ptEntry].write   = (flags & PAGING_MEM_WRITE) ? 1 : 0;
	pt[ptEntry].paddr   = paddr >> PAGE_SHIFT;
	refPhyPage(paddr);

	__native_flush_tlb_single(vaddr);
	return 0;
}

int pageUnmap(vaddr_t vaddr)
{
	uint pdEntry = vaddr >> (PD_SHIFT);
	uint ptEntry = (vaddr >> PT_SHIFT) & PTE_MASK;

	// Thank to mirroring, we can access the PD
	struct pde *pd =
	    (struct pde *)(PD_MIRROR_PAGE_IDX * (1U << 22) + PD_MIRROR_PAGE_IDX * (1U << 12));

	struct pte *pt =
	    (struct pte *)(PD_MIRROR_PAGE_IDX * (1U << 22) + pdEntry * (1U << 12));
	if (!pd[pdEntry].present)
		return -EINVAL;
	if (!pt[ptEntry].present)
		return -EINVAL;

	unrefPhyPage(pt[ptEntry].paddr << PAGE_SHIFT);
	pt[ptEntry].present = 0;

	// PTE not used. Decrease refcount on it. Is PT not used anymore ?
	if(unrefPhyPage(pd[pdEntry].pt_addr<< PT_SHIFT) == 0){
		pd[pdEntry].present = 0;
		__native_flush_tlb_single((vaddr_t)pt);
	}
	__native_flush_tlb_single(vaddr);
	return 0;

}