Removing the magic numbers from the GDT initialization (and adding a …

…TSS descriptor)
ilmmatias · Jan 21, 2025 · 682a6bf · 682a6bf
1 parent 9a8ef4e
commit 682a6bf
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Showing 4 changed files with 135 additions and 29 deletions.
diff --git a/src/boot/osloader/include/amd64/page.h b/src/boot/osloader/include/amd64/page.h
@@ -6,22 +6,25 @@
 
 #include <stdint.h>
 
-typedef struct __attribute__((packed)) {
-    uint64_t Present : 1;
-    uint64_t Writable : 1;
-    uint64_t User : 1;
-    uint64_t WriteThrough : 1;
-    uint64_t CacheDisable : 1;
-    uint64_t Accessed : 1;
-    uint64_t Dirty : 1;
-    uint64_t PageSize : 1;
-    uint64_t Global : 1;
-    uint64_t Available0 : 2;
-    uint64_t Pat : 1;
-    uint64_t Address : 40;
-    uint64_t Available1 : 7;
-    uint64_t ProtectionKey : 4;
-    uint64_t NoExecute : 1;
+typedef union {
+    struct __attribute__((packed)) {
+        uint64_t Present : 1;
+        uint64_t Writable : 1;
+        uint64_t User : 1;
+        uint64_t WriteThrough : 1;
+        uint64_t CacheDisable : 1;
+        uint64_t Accessed : 1;
+        uint64_t Dirty : 1;
+        uint64_t PageSize : 1;
+        uint64_t Global : 1;
+        uint64_t Available0 : 2;
+        uint64_t Pat : 1;
+        uint64_t Address : 40;
+        uint64_t Available1 : 7;
+        uint64_t ProtectionKey : 4;
+        uint64_t NoExecute : 1;
+    };
+    uint64_t RawData;
 } PageFrame;
 
 #endif /* _AMD64_PAGE_H_ */
diff --git a/src/kernel/hal/amd64/gdt.c b/src/kernel/hal/amd64/gdt.c
@@ -5,6 +5,40 @@
 
 extern void HalpFlushGdt(void);
 
+/*-------------------------------------------------------------------------------------------------
+ * PURPOSE:
+ *     This function initializes an entry inside the GDT.
+ *
+ * PARAMETERS:
+ *     Processor - Pointer to the processor-specific structure.
+ *     EntryOffset - Which entry to initialize.
+ *
+ * RETURN VALUE:
+ *     None.
+ *-----------------------------------------------------------------------------------------------*/
+static void InitializeEntry(
+    KeProcessor *Processor,
+    uint8_t EntryOffset,
+    uint64_t Base,
+    uint32_t Limit,
+    uint8_t Type,
+    uint8_t Dpl) {
+    HalpGdtEntry *Entry = (HalpGdtEntry *)(Processor->GdtEntries + EntryOffset);
+    Entry->LimitLow = Limit & 0xFFFF;
+    Entry->BaseLow = Base & 0xFFFFFF;
+    Entry->Type = Type;
+    Entry->Dpl = Dpl;
+    Entry->Present = 1;
+    Entry->LimitHigh = (Limit >> 16) & 0x0F;
+    Entry->System = 0;
+    Entry->LongMode = 1;
+    Entry->DefaultBig = 0;
+    Entry->Granularity = 1;
+    Entry->BaseHigh = (Base >> 24) & 0xFF;
+    Entry->BaseUpper = (Base >> 32) & 0xFFFFFFFF;
+    Entry->MustBeZero = 0;
+}
+
 /*-------------------------------------------------------------------------------------------------
  * PURPOSE:
  *     This function initializes the Global Descriptor Table. This, in combination with
@@ -17,15 +51,23 @@ extern void HalpFlushGdt(void);
  *     None.
  *-----------------------------------------------------------------------------------------------*/
 void HalpInitializeGdt(KeProcessor *Processor) {
-    Processor->GdtEntries[0] = 0x0000000000000000;
-    Processor->GdtEntries[1] = 0x00AF9A000000FFFF;
-    Processor->GdtEntries[2] = 0x00AF92000000FFFF;
-    Processor->GdtEntries[3] = 0x00AFFA000000FFFF;
-    Processor->GdtEntries[4] = 0x00AFF2000000FFFF;
+    /* The NULL descriptor should have already been zeroed out during the processor block
+     * allocation. */
+    InitializeEntry(Processor, GDT_ENTRY_KCODE, 0, 0xFFFFF, GDT_TYPE_CODE, GDT_DPL_KERNEL);
+    InitializeEntry(Processor, GDT_ENTRY_KDATA, 0, 0xFFFFF, GDT_TYPE_DATA, GDT_DPL_KERNEL);
+    InitializeEntry(Processor, GDT_ENTRY_UCODE, 0, 0xFFFFF, GDT_TYPE_CODE, GDT_DPL_USER);
+    InitializeEntry(Processor, GDT_ENTRY_UDATA, 0, 0xFFFFF, GDT_TYPE_DATA, GDT_DPL_USER);
 
-    Processor->GdtDescriptor.Limit = sizeof(Processor->GdtEntries) - 1;
-    Processor->GdtDescriptor.Base = (uint64_t)Processor->GdtEntries;
+    uint64_t TssBase = (uint64_t)&Processor->TssEntry;
+    uint16_t TssSize = sizeof(HalpTssEntry);
+    InitializeEntry(Processor, GDT_ENTRY_TSS, TssBase, TssSize, GDT_TYPE_TSS, GDT_DPL_KERNEL);
+    Processor->TssEntry.Rsp0 = (uint64_t)&Processor->SystemStack;
+    Processor->TssEntry.IoMapBase = TssSize;
 
-    __asm__ volatile("lgdt %0" : : "m"(Processor->GdtDescriptor));
+    HalpGdtDescriptor Descriptor;
+    Descriptor.Limit = sizeof(Processor->GdtEntries) - 1;
+    Descriptor.Base = (uint64_t)Processor->GdtEntries;
+    __asm__ volatile("lgdt %0" : : "m"(Descriptor));
     HalpFlushGdt();
+    __asm__ volatile("mov $0x28, %%ax; ltr %%ax" : : : "%rax");
 }
diff --git a/src/kernel/include/public/amd64/gdt.h b/src/kernel/include/public/amd64/gdt.h
@@ -0,0 +1,63 @@
+/* SPDX-FileCopyrightText: (C) 2025 ilmmatias
+ * SPDX-License-Identifier: GPL-3.0-or-later */
+
+#ifndef _AMD64_GDT_H_
+#define _AMD64_GDT_H_
+
+#include <stdint.h>
+
+#define GDT_ENTRY_NULL 0x00
+#define GDT_ENTRY_KCODE 0x08
+#define GDT_ENTRY_KDATA 0x10
+#define GDT_ENTRY_UCODE 0x18
+#define GDT_ENTRY_UDATA 0x20
+#define GDT_ENTRY_TSS 0x28
+
+#define GDT_TYPE_TSS 0x09
+#define GDT_TYPE_CODE 0x1A
+#define GDT_TYPE_DATA 0x12
+
+#define GDT_DPL_KERNEL 0x00
+#define GDT_DPL_USER 0x03
+
+typedef struct __attribute__((packed)) {
+    uint32_t Reserved0;
+    uint64_t Rsp0;
+    uint64_t Rsp1;
+    uint64_t Rsp2;
+    uint64_t Ist[8];
+    uint64_t Reserved1;
+    uint16_t Reserved2;
+    uint16_t IoMapBase;
+} HalpTssEntry;
+
+typedef union {
+    struct __attribute__((packed)) {
+        struct __attribute__((packed)) {
+            uint64_t LimitLow : 16;
+            uint64_t BaseLow : 24;
+            uint64_t Type : 5;
+            uint64_t Dpl : 2;
+            uint64_t Present : 1;
+            uint64_t LimitHigh : 4;
+            uint64_t System : 1;
+            uint64_t LongMode : 1;
+            uint64_t DefaultBig : 1;
+            uint64_t Granularity : 1;
+            uint64_t BaseHigh : 8;
+        };
+        uint32_t BaseUpper;
+        uint32_t MustBeZero;
+    };
+    struct __attribute__((packed)) {
+        uint64_t DataLow;
+        uint64_t DataHigh;
+    };
+} HalpGdtEntry;
+
+typedef struct __attribute__((packed)) {
+    uint16_t Limit;
+    uint64_t Base;
+} HalpGdtDescriptor;
+
+#endif /* _AMD64_GDT_H_ */
diff --git a/src/kernel/include/public/amd64/processor.h b/src/kernel/include/public/amd64/processor.h
@@ -4,6 +4,7 @@
 #ifndef _AMD64_PROCESSOR_H_
 #define _AMD64_PROCESSOR_H_
 
+#include <amd64/gdt.h>
 #include <ps.h>
 #include <rt/list.h>
 
@@ -20,11 +21,8 @@ typedef struct {
     RtDList DpcQueue;
     RtDList EventQueue;
     char SystemStack[8192] __attribute__((aligned(4096)));
-    uint64_t GdtEntries[5];
-    struct __attribute__((packed)) __attribute__((aligned(4))) {
-        uint16_t Limit;
-        uint64_t Base;
-    } GdtDescriptor;
+    char GdtEntries[56];
+    HalpTssEntry TssEntry;
     struct __attribute__((packed)) {
         uint16_t BaseLow;
         uint16_t Cs;