kern/amd64: cpu: Initialize GDT

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

1 files changed, 109 insertions, 0 deletions

diff --git a/src/sys/include/arch/amd64/gdt.h b/src/sys/include/arch/amd64/gdt.h
new file mode 100644
index 0000000..ee52065
--- /dev/null
+++ b/src/sys/include/arch/amd64/gdt.h
@@ -0,0 +1,109 @@
+/*
+ * 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.
+ */
+
+#ifndef _AMD64_GDT_H_
+#define _AMD64_GDT_H_
+
+#include <sys/types.h>
+#include <sys/cdefs.h>
+
+#define GDT_TSS_INDEX 5
+#define GDT_ENTRY_COUNT 7
+
+/* Segment selectors */
+#define KERNEL_CS 0x08
+#define KERNEL_DS 0x10
+#define USER_CS   0x18
+#define USER_DS   0x20
+
+/*
+ * Bit definitions for regular segment descriptors
+ *
+ * See Intel SPG 3/25 Section 3.4.5 - Segment Descriptors
+ */
+
+#define GDT_ATTRIBUTE_ACCESSED     BIT(0)  /* Accessed */
+#define GDT_ATTRIBUTE_EXECUTABLE   BIT(3)  /* Executable */
+#define GDT_ATTRIBUTE_NONSYSTEM    BIT(4)  /* Code/data */
+#define GDT_ATTRIBUTE_PRESENT      BIT(7)  /* Present */
+#define GDT_ATTRIBUTE_64BIT_CODE   BIT(13) /* 64-bit code */
+#define GDT_ATTRIBUTE_32BIT        BIT(14) /* 32-bit code/data */
+#define GDT_ATTRIBUTE_GRANULARITY  BIT(15) /* 4KiB limit granularity */
+
+/* Attributes for executable segments */
+#define GDT_ATTRIBUTE_READABLE     BIT(1)  /* Readable */
+#define GDT_ATTRIBUTE_CONFORMING   BIT(2)  /* Conforming */
+
+/* Attributes for non-executable segments */
+#define GDT_ATTRIBUTE_WRITABLE     BIT(1)  /* Writable */
+#define GDT_ATTRIBUTE_EXPANDS_DOWN BIT(2)  /* See SPG 3/25 Section 6.8.1 */
+
+/* DPL (Descriptor Privilege Level) specifier */
+#define GDT_ATTRIBUTE_DPL0         0
+#define GDT_ATTRIBUTE_DPL1         (1 << 5)
+#define GDT_ATTRIBUTE_DPL2         (2 << 5)
+#define GDT_ATTRIBUTE_DPL3         (3 << 5)
+
+struct __packed gdt_entry {
+    uint16_t limit;
+    uint16_t base_low;
+    uint8_t base_mid;
+    uint16_t attributes;
+    uint8_t base_hi;
+};
+
+struct __packed gdtr {
+    uint16_t limit;
+    uintptr_t offset;
+};
+
+extern struct gdt_entry g_gdt_data[GDT_ENTRY_COUNT];
+extern const struct gdtr g_gdtr;
+
+__always_inline static inline void
+gdt_load(void)
+{
+    __ASMV("lgdt %0\n"
+           "push %1\n"              /* Push code segment selector */
+           "lea 1f(%%rip), %%rax\n" /* Load label 1 address into RAX */
+           "push %%rax\n"           /* Push return address (label 1) */
+           "lretq\n"                /* Far return to update CS */
+           "1:\n"
+           "  mov %2, %%ax\n"       /* Load data segment selectors */
+           "  mov %%ax, %%ds\n"
+           "  mov %%ax, %%es\n"
+           "  mov %%ax, %%fs\n"
+           "  mov %%ax, %%gs\n"
+           "  mov %%ax, %%ss\n"
+           :
+           : "m" (g_gdtr), "i"(KERNEL_CS), "i"(KERNEL_DS)
+           : "rax", "memory"
+    );
+}
+#endif  /* !AMD64_GDT_H_ */