kern/amd64: Define the GDT

Signed-off-by: Ian Moffett <ian@osmora.org>

1 files changed, 98 insertions, 0 deletions

diff --git a/src/sys/arch/amd64/boot/boot_gdt.c b/src/sys/arch/amd64/boot/boot_gdt.c
new file mode 100644
index 0000000..2c1ada6
--- /dev/null
+++ b/src/sys/arch/amd64/boot/boot_gdt.c
@@ -0,0 +1,98 @@
+/*
+ * Copyright (c) 2025 Ian Marco Moffett and L5 engineers
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the project nor the names of its
+ *    contributors may be used to endorse or promote products derived from
+ *    this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <machine/gdt.h>
+
+/*
+ * The GDT should be cache line aligned, since it is accessed every time a
+ * segment selector is reloaded
+ */
+struct gdt_entry g_gdt_data[GDT_ENTRY_COUNT] = {
+    /* Null */
+    {0},
+
+    /* Kernel code (0x08) */
+    {
+        .limit      = 0x0000,
+        .base_low   = 0x0000,
+        .base_mid   = 0x00,
+        .attributes = GDT_ATTRIBUTE_64BIT_CODE | GDT_ATTRIBUTE_PRESENT   |
+                      GDT_ATTRIBUTE_DPL0       | GDT_ATTRIBUTE_NONSYSTEM |
+                      GDT_ATTRIBUTE_EXECUTABLE | GDT_ATTRIBUTE_READABLE,
+        .base_hi    = 0x00
+    },
+
+    /* Kernel data (0x10) */
+    {
+        .limit      = 0x0000,
+        .base_low   = 0x0000,
+        .base_mid   = 0x00,
+        .attributes = GDT_ATTRIBUTE_PRESENT    | GDT_ATTRIBUTE_DPL0      |
+                      GDT_ATTRIBUTE_NONSYSTEM  | GDT_ATTRIBUTE_WRITABLE,
+        .base_hi    = 0x00
+    },
+
+    /* User code (0x18) */
+    {
+        .limit      = 0x0000,
+        .base_low   = 0x0000,
+        .base_mid   = 0x00,
+        .attributes = GDT_ATTRIBUTE_64BIT_CODE | GDT_ATTRIBUTE_PRESENT   |
+                      GDT_ATTRIBUTE_DPL3       | GDT_ATTRIBUTE_NONSYSTEM |
+                      GDT_ATTRIBUTE_EXECUTABLE | GDT_ATTRIBUTE_READABLE,
+        .base_hi    = 0x00
+    },
+
+    /* User data (0x20) */
+    {
+        .limit      = 0x0000,
+        .base_low   = 0x0000,
+        .base_mid   = 0x00,
+        .attributes = GDT_ATTRIBUTE_PRESENT    | GDT_ATTRIBUTE_DPL3      |
+                      GDT_ATTRIBUTE_NONSYSTEM  | GDT_ATTRIBUTE_WRITABLE,
+        .base_hi    = 0x00
+    },
+
+    /*
+     * TSS segment (0x28)
+     *
+     * NOTE: 64-bit TSS descriptors are 16 bytes, equivalent to the size of two
+     *       regular descriptor entries.
+     *       See Intel SPG 3/25 Section 9.2.3 - TSS Descriptor in 64-bit mode.
+     */
+    {0}, {0}
+};
+
+/* Verify that the GDT is of the correct size */
+__static_assert(sizeof(g_gdt_data) == (8 * GDT_ENTRY_COUNT));
+
+const struct gdtr g_gdtr = {
+    .limit = sizeof(g_gdt_data) - 1,
+    .offset = (uintptr_t)&g_gdt_data[0]
+};