kernel/amd64: Improve GDT implementation

* Switched to using macros for the GDT entry bits, to improve clarity
  and make the code easier to modify. Also got rid of some junk values
  in a few of the entries.
* Shrunk the GDT data array from 256 entries to 7 (GDT_ENTRY_COUNT)
  since the GDT will only ever use 7 of the entries for now.
* Aligned the GDT using __cacheline_aligned, because the GDT entries
  will be read every time a segment selector is loaded.
* Modified code dealing with the GDTR to just use `g_gdtr`, since the
  same GDT/GDTR is always used.
* In `gdt_load()`, changed `eax` -> `ax`, since segment registers are
  only 16 bits. Also replaced the `gdtr` to just use `g_gdtr`.
  Additionally, used the inline assembly formatting to supply segment
  selectors with KERNEL_CS/DS macros.

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

1 files changed, 50 insertions, 19 deletions

diff --git a/sys/include/arch/amd64/gdt.h b/sys/include/arch/amd64/gdt.h
index 55666a7..0c5faf1 100644
--- a/sys/include/arch/amd64/gdt.h
+++ b/sys/include/arch/amd64/gdt.h
@@ -4,18 +4,48 @@
 #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
-#define GDT_TSS 5
+#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;
-    uint8_t access;
-    uint8_t granularity;
+    uint16_t attributes;
     uint8_t base_hi;
 };
 
@@ -24,27 +54,28 @@ struct __packed gdtr {
     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(struct gdtr *gdtr)
+gdt_load(void)
 {
     __ASMV("lgdt %0\n"
-           "push $8\n"               /* Push CS */
-           "lea 1f(%%rip), %%rax\n"  /* Load 1 label address into RAX */
-           "push %%rax\n"            /* Push the return address (label 1) */
-           "lretq\n"                 /* Far return to update CS */
+           "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 $0x10, %%eax\n"
-           "  mov %%eax, %%ds\n"
-           "  mov %%eax, %%es\n"
-           "  mov %%eax, %%fs\n"
-           "  mov %%eax, %%gs\n"
-           "  mov %%eax, %%ss\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" (*gdtr)
+           : "m" (g_gdtr), "i"(KERNEL_CS), "i"(KERNEL_DS)
            : "rax", "memory"
     );
 }
 
-extern struct gdt_entry g_gdt_data[256];
-
 #endif  /* !AMD64_GDT_H_ */