147 lines
5 KiB
C
147 lines
5 KiB
C
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#include "userprog/gdt.h"
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#include <debug.h>
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#include "userprog/tss.h"
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#include "threads/palloc.h"
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#include "threads/vaddr.h"
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/* The Global Descriptor Table (GDT).
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The GDT, an x86-specific structure, defines segments that can
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potentially be used by all processes in a system, subject to
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their permissions. There is also a per-process Local
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Descriptor Table (LDT) but that is not used by modern
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operating systems.
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Each entry in the GDT, which is known by its byte offset in
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the table, identifies a segment. For our purposes only three
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types of segments are of interest: code, data, and TSS or
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Task-State Segment descriptors. The former two types are
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exactly what they sound like. The TSS is used primarily for
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stack switching on interrupts.
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For more information on the GDT as used here, refer to
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[IA32-v3a] 3.2 "Using Segments" through 3.5 "System Descriptor
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Types". */
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static uint64_t gdt[SEL_CNT];
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/* GDT helpers. */
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static uint64_t make_code_desc (int dpl);
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static uint64_t make_data_desc (int dpl);
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static uint64_t make_tss_desc (void *laddr);
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static uint64_t make_gdtr_operand (uint16_t limit, void *base);
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/* Sets up a proper GDT. The bootstrap loader's GDT didn't
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include user-mode selectors or a TSS, but we need both now. */
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void
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gdt_init (void)
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{
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uint64_t gdtr_operand;
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/* Initialize GDT. */
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gdt[SEL_NULL / sizeof *gdt] = 0;
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gdt[SEL_KCSEG / sizeof *gdt] = make_code_desc (0);
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gdt[SEL_KDSEG / sizeof *gdt] = make_data_desc (0);
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gdt[SEL_UCSEG / sizeof *gdt] = make_code_desc (3);
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gdt[SEL_UDSEG / sizeof *gdt] = make_data_desc (3);
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gdt[SEL_TSS / sizeof *gdt] = make_tss_desc (tss_get ());
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/* Load GDTR, TR. See [IA32-v3a] 2.4.1 "Global Descriptor
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Table Register (GDTR)", 2.4.4 "Task Register (TR)", and
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6.2.4 "Task Register". */
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gdtr_operand = make_gdtr_operand (sizeof gdt - 1, gdt);
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asm volatile ("lgdt %0" : : "m" (gdtr_operand));
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asm volatile ("ltr %w0" : : "q" (SEL_TSS));
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}
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/* System segment or code/data segment? */
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enum seg_class
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{
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CLS_SYSTEM = 0, /* System segment. */
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CLS_CODE_DATA = 1 /* Code or data segment. */
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};
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/* Limit has byte or 4 kB page granularity? */
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enum seg_granularity
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{
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GRAN_BYTE = 0, /* Limit has 1-byte granularity. */
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GRAN_PAGE = 1 /* Limit has 4 kB granularity. */
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};
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/* Returns a segment descriptor with the given 32-bit BASE and
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20-bit LIMIT (whose interpretation depends on GRANULARITY).
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The descriptor represents a system or code/data segment
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according to CLASS, and TYPE is its type (whose interpretation
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depends on the class).
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The segment has descriptor privilege level DPL, meaning that
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it can be used in rings numbered DPL or lower. In practice,
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DPL==3 means that user processes can use the segment and
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DPL==0 means that only the kernel can use the segment. See
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[IA32-v3a] 4.5 "Privilege Levels" for further discussion. */
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static uint64_t
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make_seg_desc (uint32_t base,
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uint32_t limit,
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enum seg_class class,
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int type,
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int dpl,
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enum seg_granularity granularity)
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{
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uint32_t e0, e1;
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ASSERT (limit <= 0xfffff);
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ASSERT (class == CLS_SYSTEM || class == CLS_CODE_DATA);
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ASSERT (type >= 0 && type <= 15);
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ASSERT (dpl >= 0 && dpl <= 3);
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ASSERT (granularity == GRAN_BYTE || granularity == GRAN_PAGE);
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e0 = ((limit & 0xffff) /* Limit 15:0. */
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| (base << 16)); /* Base 15:0. */
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e1 = (((base >> 16) & 0xff) /* Base 23:16. */
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| (type << 8) /* Segment type. */
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| (class << 12) /* 0=system, 1=code/data. */
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| (dpl << 13) /* Descriptor privilege. */
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| (1 << 15) /* Present. */
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| (limit & 0xf0000) /* Limit 16:19. */
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| (1 << 22) /* 32-bit segment. */
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| (granularity << 23) /* Byte/page granularity. */
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| (base & 0xff000000)); /* Base 31:24. */
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return e0 | ((uint64_t) e1 << 32);
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}
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/* Returns a descriptor for a readable code segment with base at
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0, a limit of 4 GB, and the given DPL. */
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static uint64_t
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make_code_desc (int dpl)
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{
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return make_seg_desc (0, 0xfffff, CLS_CODE_DATA, 10, dpl, GRAN_PAGE);
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}
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/* Returns a descriptor for a writable data segment with base at
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0, a limit of 4 GB, and the given DPL. */
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static uint64_t
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make_data_desc (int dpl)
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{
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return make_seg_desc (0, 0xfffff, CLS_CODE_DATA, 2, dpl, GRAN_PAGE);
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}
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/* Returns a descriptor for an "available" 32-bit Task-State
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Segment with its base at the given linear address, a limit of
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0x67 bytes (the size of a 32-bit TSS), and a DPL of 0.
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See [IA32-v3a] 6.2.2 "TSS Descriptor". */
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static uint64_t
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make_tss_desc (void *laddr)
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{
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return make_seg_desc ((uint32_t) laddr, 0x67, CLS_SYSTEM, 9, 0, GRAN_BYTE);
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}
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/* Returns a descriptor that yields the given LIMIT and BASE when
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used as an operand for the LGDT instruction. */
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static uint64_t
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make_gdtr_operand (uint16_t limit, void *base)
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{
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return limit | ((uint64_t) (uint32_t) base << 16);
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}
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