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.

1 files changed, 33 insertions, 0 deletions

diff --git a/src/kernel/arch/amd64/boot64.S b/src/kernel/arch/amd64/boot64.S
new file mode 100644
index 0000000..f7ec60e
--- /dev/null
+++ b/src/kernel/arch/amd64/boot64.S
@@ -0,0 +1,33 @@
+.global boot64
+boot64:
+	lgdt (GdtPointer) // try reloading gdt again
+	mov $(6 << 3 | 3), %ax // SEG_TSS
+	ltr %ax
+
+	push %rdi // preserve multiboot struct
+	call sysenter_setup
+	pop %rdi
+
+	// multiboot struct in %rdi
+	jmp kmain_early
+
+
+.section .shared
+/* https://wiki.osdev.org/Task_State_Segment#Long_Mode */
+.global TSS
+.align 8
+TSS:
+	.4byte 0 /* reserved */
+	.rept 3
+	.8byte _isr_mini_stack /* stacks for privilege level changes */
+	.endr
+	.8byte 0 /* reserved */
+	.rept 7
+	.8byte _isr_mini_stack /* IST - stack pointer loaded for interrupts */
+	.endr
+	.8byte 0 /* reserved */
+	.4byte 0 /* reserved + IOPB (unused) */
+	.if . - TSS != 104
+		.error "bad tss size"
+		.abort
+	.endif