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