kernel: set up the GDT in assembly

This is just for simplicity's sake.

I think I could even omit the `movw $TSS, (GdtTss + 2)` and have the linker
fill that out as a relocation, but that would probably be more complex overall.

2 files changed, 35 insertions, 113 deletions

diff --git a/src/kernel/arch/amd64/32/boot.s b/src/kernel/arch/amd64/32/boot.S
index b0808d3..3f5798f 100644
--- a/src/kernel/arch/amd64/32/boot.s
+++ b/src/kernel/arch/amd64/32/boot.S
@@ -34,8 +34,9 @@ _start:
 	and $(~4), %eax // disable coprocessor emulation
 	mov %eax, %cr0
 
-	call gdt_init
-	lgdt (lgdt_arg)
+	/* fill out the TSS's address */
+	movw $TSS, (GdtTss + 2)
+	lgdt (GdtPointer)
 
 	pop %edi
 
@@ -79,3 +80,35 @@ cpu_pause:
 	hlt
 	cli
 	ret
+
+.section .shared
+.global GDT
+.align 8
+GDT:
+#define GdtLimit(p)   (p & 0xFFFF) | ((p >> 16) << 48) /* doesn't check if p fits in 20 bits */
+#define GdtAccessed   (1<<40)
+#define GdtReadWrite  (1<<41)
+#define GdtCode       (1<<43)
+#define GdtCodeOrData (1<<44)
+#define GdtRing(r)    (r<<45)
+#define GdtPresent    (1<<47)
+#define GdtAvailable  (1<<52)
+#define GdtLongMode   (1<<53)
+#define GdtPageGran   (1<<55) /* limit is in pages */
+#define GdtCommon     (GdtLimit(0xFFFFF) | GdtPageGran | GdtAccessed | \
+                       GdtReadWrite | GdtCodeOrData | GdtPresent | GdtAvailable)
+.8byte 0                                              /* [0] = SEG_null */
+.8byte GdtCommon | GdtRing(0) | GdtLongMode | GdtCode /* [1] = SEG_r0code */
+.8byte GdtCommon | GdtRing(0) | GdtLongMode           /* [2] = SEG_r0data */
+.8byte GdtCommon | GdtRing(3)               | GdtCode /* [3] = SEG_r3code32 */
+.8byte GdtCommon | GdtRing(3) | GdtLongMode           /* [4] = SEG_r3data */
+.8byte GdtCommon | GdtRing(3) | GdtLongMode | GdtCode /* [5] = SEG_r3code */
+GdtTss:
+.8byte GdtLimit(104) | (9<<40) | GdtPresent | GdtAvailable /* [6] = SEG_TSS */
+.8byte 0                                                   /* [7] = SEG_TSS2 */
+
+.section .data
+.global GdtPointer
+GdtPointer:
+.2byte 63  /* size of the GDT - 1 */
+.4byte GDT /* address of the GDT */
diff --git a/src/kernel/arch/amd64/32/gdt.c b/src/kernel/arch/amd64/32/gdt.c
deleted file mode 100644
index e643f27..0000000
--- a/src/kernel/arch/amd64/32/gdt.c
+++ /dev/null
@@ -1,111 +0,0 @@
-#include <kernel/arch/amd64/32/util.h>
-#include <kernel/arch/amd64/boot.h>
-#include <stdbool.h>
-#include <stdint.h>
-
-extern char _isr_mini_stack;
-
-struct gdt_entry {
-	uint64_t limit_low  : 16;
-	uint64_t base_low   : 24;
-	uint64_t accessed   :  1; // set by the processor
-	                          // CODE        | DATA
-	uint64_t rw         :  1; // readable?   | writeable?
-	uint64_t conforming :  1; // conforming? | expands down?
-	uint64_t code       :  1; // 1           | 0
-
-	uint64_t codeordata :  1; // 1 for everything other than TSS and LDT
-	uint64_t ring       :  2;
-	uint64_t present    :  1; // always 1
-	uint64_t limit_high :  4;
-	uint64_t available  :  1; // ???
-	uint64_t long_mode  :  1;
-	uint64_t x32        :  1;
-	uint64_t gran       :  1; // 1 - 4kb, 0 - 1b
-	uint64_t base_high  :  8;
-} __attribute__((packed));
-
-struct tss_entry {
-	uint32_t reserved0;
-	uint64_t rsp[3];
-	uint64_t ist[8];
-	uint64_t reserved2;
-	uint16_t reserved3;
-	uint16_t iopb;
-} __attribute__((packed));
-
-struct lgdt_arg {
-	uint16_t limit;
-	uint32_t base;
-} __attribute__((packed));
-
-__attribute__((section(".shared")))
-static struct gdt_entry GDT[SEG_end];
-__attribute__((section(".shared")))
-static struct tss_entry TSS;
-
-struct lgdt_arg lgdt_arg;
-
-static void gdt_fillout(struct gdt_entry* entry, uint8_t ring, bool code) {
-	*entry = (struct gdt_entry) {
-		// set up the identity mapping
-		.limit_low  = 0xFFFF,
-		.limit_high = 0xF,
-		.gran       = 1,
-		.base_low   = 0,
-		.base_high  = 0,
-
-		.ring       = ring,
-		.code       = code,
-
-		.accessed   = 0,
-		.rw         = 1,
-		.conforming = 0,
-		.codeordata = 1,
-		.present    = 1,
-		.long_mode  = 1,
-		.available  = 1,
-		.x32        = 0,
-	};
-}
-
-#pragma GCC diagnostic ignored "-Wpointer-to-int-cast"
-void gdt_init(void) {
-	GDT[SEG_null].present = 0;
-
-	gdt_fillout(&GDT[SEG_r0code], 0, true);
-	gdt_fillout(&GDT[SEG_r0data], 0, false);
-	gdt_fillout(&GDT[SEG_r3code32], 3, true);
-	gdt_fillout(&GDT[SEG_r3data], 3, false);
-	gdt_fillout(&GDT[SEG_r3code], 3, true);
-
-	lgdt_arg.limit = sizeof(GDT) - 1;
-	lgdt_arg.base = (uint64_t)&GDT;
-
-
-	memset32(&TSS, 0, sizeof(TSS));
-	for (int i = 0; i < 3; i++)
-		TSS.rsp[i] = (uint64_t)&_isr_mini_stack;
-	TSS.ist[1] = (uint64_t)&_isr_mini_stack;
-
-	uint64_t tss_addr = (uint64_t)&TSS;
-	GDT[SEG_TSS] = (struct gdt_entry) {
-		.limit_low  = sizeof(TSS),
-		.limit_high = sizeof(TSS) >> 16,
-		.gran       = 0,
-		.base_low   = tss_addr,
-		.base_high  = tss_addr >> 24,
-
-		.accessed   = 1,
-		.rw         = 0,
-		.conforming = 0,
-		.code       = 1,
-		.codeordata = 0,
-		.ring       = 0, // was 3 pre-port
-		.present    = 1,
-		.available  = 1,
-		.long_mode  = 0,
-		.x32        = 0,
-	};
-	memset32(&GDT[SEG_TSS2], 0, sizeof GDT[SEG_TSS2]);
-}