157 lines
5.0 KiB
C
157 lines
5.0 KiB
C
#include <kernel/x86/gdt.h>
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#include <stdint.h>
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#include <stdio.h>
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#ifdef __TESTING__
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#include <kernel/_kernel.h>
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#endif
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/**
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* What is this file?
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*
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* Well, to properly set up a lot of the system, we need something called a GDT
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* or, a Global Descriptor Table.
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*
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* This table, establishes a few things.
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*
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* Mainly it sets 4 segments,
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* A kernel code segment, with RING 0 permissions
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* A kernel data segment, with RING 0 permissions
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* A user code segment, with RING 3 permissions
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* A user data segment, with RING 3 permissions
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*
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* This allows for future userspace to properly segment code and data,
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* anything in userspace shouldn't have access to hardware like the kernel does
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* So by passing through this GDT, we can dish out authority to access certain data, functions,
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* etc, by going through the CPU permission system (RING 0 - 3)
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*/
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#define GDT_SIZE 5
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// Each define here is for a specific flag in the descriptor.
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// Refer to the intel documentation for a description of what each one does.
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#define SEG_DESCTYPE(x) ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
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#define SEG_PRES(x) ((x) << 0x07) // Present
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#define SEG_SAVL(x) ((x) << 0x0C) // Available for system use
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#define SEG_LONG(x) ((x) << 0x0D) // Long mode
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#define SEG_SIZE(x) ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
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#define SEG_GRAN(x) ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
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#define SEG_PRIV(x) (((x) & 0x03) << 0x05) // Set privilege level (0 - 3)
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#define SEG_DATA_RD 0x00 // Read-Only
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#define SEG_DATA_RDA 0x01 // Read-Only, accessed
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#define SEG_DATA_RDWR 0x02 // Read/Write
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#define SEG_DATA_RDWRA 0x03 // Read/Write, accessed
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#define SEG_DATA_RDEXPD 0x04 // Read-Only, expand-down
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#define SEG_DATA_RDEXPDA 0x05 // Read-Only, expand-down, accessed
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#define SEG_DATA_RDWREXPD 0x06 // Read/Write, expand-down
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#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
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#define SEG_CODE_EX 0x08 // Execute-Only
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#define SEG_CODE_EXA 0x09 // Execute-Only, accessed
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#define SEG_CODE_EXRD 0x0A // Execute/Read
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#define SEG_CODE_EXRDA 0x0B // Execute/Read, accessed
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#define SEG_CODE_EXC 0x0C // Execute-Only, conforming
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#define SEG_CODE_EXCA 0x0D // Execute-Only, conforming, accessed
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#define SEG_CODE_EXRDC 0x0E // Execute/Read, conforming
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#define SEG_CODE_EXRDCA 0x0F // Execute/Read, conforming, accessed
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#define GDT_CODE_PL0 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
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SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
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SEG_PRIV(0) | SEG_CODE_EXRD
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#define GDT_DATA_PL0 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
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SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
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SEG_PRIV(0) | SEG_DATA_RDWR
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#define GDT_CODE_PL3 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
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SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
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SEG_PRIV(3) | SEG_CODE_EXRD
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#define GDT_DATA_PL3 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
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SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
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SEG_PRIV(3) | SEG_DATA_RDWR
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uint64_t gdt[GDT_SIZE];
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void set_gdt(unsigned short limit, uint64_t* base);
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void reload_segments(void);
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uint64_t create_descriptor(uint32_t base, uint32_t limit, uint16_t flag)
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{
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uint64_t descriptor;
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descriptor = limit & 0x000F0000;
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descriptor |= (flag << 8) & 0x00F0FF00;
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descriptor |= (base >> 16) & 0x000000FF;
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descriptor |= base & 0xFF000000;
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descriptor <<= 32;
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descriptor |= base << 16;
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descriptor |= limit & 0x0000FFFF;
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return descriptor;
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}
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#ifdef __TESTING__
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void dump_gdt(void)
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{
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for (int i = 0; i < GDT_SIZE; i++) {
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printf("GDT_ENTRY %d: %4 | %2\n", i, gdt[i], gdt[i]);
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}
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}
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#endif
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void gdt_init(void)
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{
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#ifdef __TESTING__
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kinfo("Initializing the GDT");
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#endif
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gdt[0] = create_descriptor(0, 0, 0); // null
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gdt[1] = create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL0)); // kernel code
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gdt[2] = create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL0)); // kernel data
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gdt[3] = create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL3)); // user code
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gdt[4] = create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL3)); // user data
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set_gdt((sizeof(uint64_t) * GDT_SIZE) - 1, &(gdt[0])); // limit, base
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reload_segments();
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#ifdef __TESTING__
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kinfo("Initialized the GDT");
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#endif
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#ifdef __TESTING__
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dump_gdt();
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#endif
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}
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#undef SEG_DESCTYPE
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#undef SEG_PRES
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#undef SEG_SAVL
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#undef SEG_LONG
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#undef SEG_SIZE
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#undef SEG_GRAN
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#undef SEG_PRIV
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#undef SEG_DATA_RD
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#undef SEG_DATA_RDA
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#undef SEG_DATA_RDWR
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#undef SEG_DATA_RDWRA
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#undef SEG_DATA_RDEXPD
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#undef SEG_DATA_RDEXPDA
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#undef SEG_DATA_RDWREXPD
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#undef SEG_DATA_RDWREXPDA
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#undef SEG_CODE_EX
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#undef SEG_CODE_EXA
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#undef SEG_CODE_EXRD
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#undef SEG_CODE_EXRDA
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#undef SEG_CODE_EXC
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#undef SEG_CODE_EXCA
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#undef SEG_CODE_EXRDC
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#undef SEG_CODE_EXRDCA
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#undef GDT_CODE_PL0
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#undef GDT_DATA_PL0
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#undef GDT_CODE_PL3
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#undef GDT_DATA_PL3
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#undef GDT_SIZE
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