From 87c135b010b9d4ee35c362076daa734b451049dc Mon Sep 17 00:00:00 2001
From: Josh Holtrop <jholtrop@gmail.com>
Date: Sun, 25 Oct 2020 16:49:09 -0400
Subject: [PATCH] set up a GDT to stop using the one GRUB set up

---
 src/gdt.S      | 16 ++++++++++++++++
 src/gdt.c      | 20 ++++++++++++++++++++
 src/gdt.h      |  9 +++++----
 src/hos_main.c |  2 ++
 4 files changed, 43 insertions(+), 4 deletions(-)
 create mode 100644 src/gdt.S
 create mode 100644 src/gdt.c

diff --git a/src/gdt.S b/src/gdt.S
new file mode 100644
index 0000000..b0dcfb9
--- /dev/null
+++ b/src/gdt.S
@@ -0,0 +1,16 @@
+.global gdt_set
+.extern gdtr
+.type gdt_set, @function
+gdt_set:
+    lgdt gdtr
+    jmp $0x8, $gdt_set_reload
+gdt_set_reload:
+    mov $0x10, %ax
+    mov %ax, %ds
+    mov %ax, %es
+    mov %ax, %fs
+    mov %ax, %gs
+    mov %ax, %ss
+    ret
+
+.size gdt_set, . - gdt_set
diff --git a/src/gdt.c b/src/gdt.c
new file mode 100644
index 0000000..0bba535
--- /dev/null
+++ b/src/gdt.c
@@ -0,0 +1,20 @@
+#include "gdt.h"
+
+static const gdt_entry_t gdt_entries[] = {
+    /* Null descriptor */
+    0u,
+    /* Code segment for kernel */
+    gdt_build_entry(0u, 0xFFFFFu, 1u, 0u, 1u, 1u, 0u, 1u, 0u, 1u, 1u, 0u),
+    /* Data segment for kernel */
+    gdt_build_entry(0u, 0xFFFFFu, 1u, 0u, 1u, 0u, 0u, 1u, 0u, 1u, 1u, 0u),
+};
+
+gdtr_t gdtr;
+
+void gdt_init(void)
+{
+    gdtr.size = sizeof(gdt_entries);
+    gdtr.offset_lower = (uintptr_t)gdt_entries & 0xFFFFu;
+    gdtr.offset_upper = (uintptr_t)gdt_entries >> 16u;
+    gdt_set();
+}
diff --git a/src/gdt.h b/src/gdt.h
index 24732f1..ef3bcc8 100644
--- a/src/gdt.h
+++ b/src/gdt.h
@@ -5,15 +5,13 @@
 
 typedef struct {
     uint16_t size;
-    uint16_t offset_upper;
     uint16_t offset_lower;
+    uint16_t offset_upper;
     uint16_t _reserved;
 } gdtr_t;
-#define gdt_build_gdtr(size, offset) \
-    {(size), ((offset >> 16u) & 0xFFFFu), (offset & 0xFFFFu), 0u}
 
 typedef uint64_t gdt_entry_t;
-#define gdt_build_entry(base, limit, p, privl, s, ex, dc, rw, ac, gr, sz, l) \
+#define gdt_build_entry(base, limit, pr, privl, s, ex, dc, rw, ac, gr, sz, l) \
     (gdt_entry_t)( \
             (((gdt_entry_t)base << 32) & 0xFF00000000000000ull) | /* Base 0:15 */ \
             (((gdt_entry_t)gr & 0x1u) << 55) | /* Granularity (0 = bytes, 1 = blocks) */ \
@@ -30,4 +28,7 @@ typedef uint64_t gdt_entry_t;
             (((gdt_entry_t)base << 16) & 0x000000FFFFFF0000ull) | /* Base 0:23 */ \
              ((gdt_entry_t)limit & 0x000000000000FFFFull)) /* Limit 0:15 */
 
+void gdt_init(void);
+void gdt_set(void);
+
 #endif
diff --git a/src/hos_main.c b/src/hos_main.c
index d002f47..0582d05 100644
--- a/src/hos_main.c
+++ b/src/hos_main.c
@@ -2,9 +2,11 @@
 #include "fb.h"
 #include "mbinfo.h"
 #include "klog.h"
+#include "gdt.h"
 
 void hos_main(uint32_t mbinfo_addr)
 {
+    gdt_init();
     if (!mbinfo_init(mbinfo_addr))
     {
         return;