From d1ff7bcc91886626dc9060ec5fb67ee102ab7c1d Mon Sep 17 00:00:00 2001
From: Jon Santmyer <jon@jonsantmyer.com>
Date: Mon, 11 Mar 2024 21:30:31 -0400
Subject: usermode capable kernel with logging syscall

---
 arch/x86_64/gdt.c | 125 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)
 create mode 100644 arch/x86_64/gdt.c

(limited to 'arch/x86_64/gdt.c')

diff --git a/arch/x86_64/gdt.c b/arch/x86_64/gdt.c
new file mode 100644
index 0000000..d996dcb
--- /dev/null
+++ b/arch/x86_64/gdt.c
@@ -0,0 +1,125 @@
+#include "tables.h"
+#include "tss.h"
+
+enum
+{
+    GDT_SEGMENT_KERNEL_NULL = 0,
+    GDT_SEGMENT_KERNEL_CODE,
+    GDT_SEGMENT_KERNEL_DATA,
+
+    GDT_SEGMENT_USER_NULL,
+    GDT_SEGMENT_USER_DATA,
+    GDT_SEGMENT_USER_CODE,
+
+    GDT_SEGMENT_TSS_LOW,
+    GDT_SEGMENT_TSS_HIGH,
+
+    GDT_SEGMENT_COUNT
+};
+
+__attribute__((aligned(0x1000)))
+static struct SegmentDescriptor s_gdtd[GDT_SEGMENT_COUNT] =
+{
+    { 0 },  /* Kernel NULL */
+    {       /* Kernel Code (64-bit RO EX DPL0) */
+        .limit_0_15 = 0xFFFF,
+        .base_0_15 = 0,
+        .base_16_23 = 0,
+        .type = CD_SEGMENT_TYPE_CODE | CD_SEGMENT_TYPE_CODE_CONFORMING,
+        .s = 1,
+        .dpl = 0,
+        .p = 1,
+        .limit_16_19 = 0xF,
+        .l = 1,
+        .d_b = 0,
+        .g = 0,
+        .base_24_31 = 0
+    },
+    {       /* Kernel Data (64-bit RW DPL0) */
+        .limit_0_15 = 0xFFFF,
+        .base_0_15 = 0,
+        .base_16_23 = 0,
+        .type = CD_SEGMENT_TYPE_WRITEABLE,
+        .s = 1,
+        .dpl = 0,
+        .p = 1,
+        .limit_16_19 = 0xF,
+        .l = 1,
+        .d_b = 0,
+        .g = 0,
+        .base_24_31 = 0
+    },
+    { 0 },  /* User NULL */
+    {       /* User Data (64-bit RO EX DPL3)*/
+        .limit_0_15 = 0xFFFF,
+        .base_0_15 = 0,
+        .base_16_23 = 0,
+        .type = CD_SEGMENT_TYPE_WRITEABLE,
+        .s = 1,
+        .dpl = 3,
+        .p = 1,
+        .limit_16_19 = 0xF,
+        .l = 1,
+        .d_b = 0,
+        .g = 0,
+        .base_24_31 = 0,
+    },
+    {       /* User Code (64-bit RO EX DPL3)*/
+        .limit_0_15 = 0xFFFF,
+        .base_0_15 = 0,
+        .base_16_23 = 0,
+        .type = CD_SEGMENT_TYPE_CODE | CD_SEGMENT_TYPE_CODE_CONFORMING,
+        .s = 1,
+        .dpl = 3,
+        .p = 1,
+        .limit_16_19 = 0xF,
+        .l = 1,
+        .d_b = 0,
+        .g = 0,
+        .base_24_31 = 0,
+    },
+    {       /* TSS Low */
+        .limit_0_15 = sizeof(struct TSS),
+        .base_0_15 = 0,
+        .base_16_23 = 0,
+        .type = S_SEGMENT_TYPE_TSS_AVAIL,
+        .avl = 1,
+        .s = 0,
+        .dpl = 0,
+        .p = 1,
+        .limit_16_19 = 0,
+        .l = 1,
+        .d_b = 0,
+        .g = 0,
+        .base_24_31 = 0,
+    },
+    { 0 }
+};
+static struct XDTR s_gdtr = {
+    .length = sizeof(s_gdtd) - 1,
+    .address = (uintptr_t)&s_gdtd
+};
+
+static struct TSS s_tss = {
+    
+};
+
+extern void x86_64_lgdt(struct XDTR *gdtr);
+extern void x86_64_flush_tss(void);
+void
+x86_64_load_gdt(void)
+{
+    {
+        struct SegmentDescriptor *tss_lo = &s_gdtd[GDT_SEGMENT_TSS_LOW];
+        struct SegmentDescriptor *tss_hi = &s_gdtd[GDT_SEGMENT_TSS_HIGH];
+        uintptr_t tssb = (uintptr_t)&s_tss;
+        tss_lo->base_0_15 = tssb & 0xFFFF;
+        tss_lo->base_16_23 = (tssb >> 16) & 0xFF;
+        tss_lo->base_24_31 = (tssb >> 24) & 0xFF;
+        tss_hi->limit_0_15 = (tssb >> 32) & 0xFFFF;
+        tss_hi->base_0_15 = (tssb >> 48) & 0xFFFF;
+    }
+
+    x86_64_lgdt(&s_gdtr);
+    x86_64_flush_tss();
+}
-- 
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