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99 lines
3.2 KiB
99 lines
3.2 KiB
#include <defs.h>
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#include <x86.h>
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#include <mmu.h>
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#include <memlayout.h>
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#include <pmm.h>
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/* *
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* Task State Segment:
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*
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* The TSS may reside anywhere in memory. A special segment register called
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* the Task Register (TR) holds a segment selector that points a valid TSS
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* segment descriptor which resides in the GDT. Therefore, to use a TSS
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* the following must be done in function gdt_init:
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* - create a TSS descriptor entry in GDT
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* - add enough information to the TSS in memory as needed
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* - load the TR register with a segment selector for that segment
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*
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* There are several fileds in TSS for specifying the new stack pointer when a
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* privilege level change happens. But only the fields SS0 and ESP0 are useful
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* in our os kernel.
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*
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* The field SS0 contains the stack segment selector for CPL = 0, and the ESP0
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* contains the new ESP value for CPL = 0. When an interrupt happens in protected
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* mode, the x86 CPU will look in the TSS for SS0 and ESP0 and load their value
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* into SS and ESP respectively.
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* */
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static struct taskstate ts = {0};
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/* *
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* Global Descriptor Table:
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*
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* The kernel and user segments are identical (except for the DPL). To load
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* the %ss register, the CPL must equal the DPL. Thus, we must duplicate the
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* segments for the user and the kernel. Defined as follows:
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* - 0x0 : unused (always faults -- for trapping NULL far pointers)
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* - 0x8 : kernel code segment
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* - 0x10: kernel data segment
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* - 0x18: user code segment
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* - 0x20: user data segment
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* - 0x28: defined for tss, initialized in gdt_init
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* */
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static struct segdesc gdt[] = {
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SEG_NULL,
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[SEG_KTEXT] = SEG(STA_X | STA_R, 0x0, 0xFFFFFFFF, DPL_KERNEL),
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[SEG_KDATA] = SEG(STA_W, 0x0, 0xFFFFFFFF, DPL_KERNEL),
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[SEG_UTEXT] = SEG(STA_X | STA_R, 0x0, 0xFFFFFFFF, DPL_USER),
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[SEG_UDATA] = SEG(STA_W, 0x0, 0xFFFFFFFF, DPL_USER),
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[SEG_TSS] = SEG_NULL,
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};
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static struct pseudodesc gdt_pd = {
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sizeof(gdt) - 1, (uint32_t)gdt
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};
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/* *
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* lgdt - load the global descriptor table register and reset the
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* data/code segement registers for kernel.
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* */
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static inline void
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lgdt(struct pseudodesc *pd) {
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asm volatile ("lgdt (%0)" :: "r" (pd));
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asm volatile ("movw %%ax, %%gs" :: "a" (USER_DS));
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asm volatile ("movw %%ax, %%fs" :: "a" (USER_DS));
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asm volatile ("movw %%ax, %%es" :: "a" (KERNEL_DS));
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asm volatile ("movw %%ax, %%ds" :: "a" (KERNEL_DS));
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asm volatile ("movw %%ax, %%ss" :: "a" (KERNEL_DS));
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// reload cs
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asm volatile ("ljmp %0, $1f\n 1:\n" :: "i" (KERNEL_CS));
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}
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/* temporary kernel stack */
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uint8_t stack0[1024];
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/* gdt_init - initialize the default GDT and TSS */
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static void
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gdt_init(void) {
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// Setup a TSS so that we can get the right stack when we trap from
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// user to the kernel. But not safe here, it's only a temporary value,
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// it will be set to KSTACKTOP in lab2.
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ts.ts_esp0 = (uint32_t)&stack0 + sizeof(stack0);
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ts.ts_ss0 = KERNEL_DS;
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// initialize the TSS filed of the gdt
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gdt[SEG_TSS] = SEG16(STS_T32A, (uint32_t)&ts, sizeof(ts), DPL_KERNEL);
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gdt[SEG_TSS].sd_s = 0;
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// reload all segment registers
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lgdt(&gdt_pd);
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// load the TSS
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ltr(GD_TSS);
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}
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/* pmm_init - initialize the physical memory management */
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void
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pmm_init(void) {
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gdt_init();
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}
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