From acf8f2a2a3894304fa7e9e48f4fb104b65298f12 Mon Sep 17 00:00:00 2001
From: Gunnar Beutner <gbeutner@serenityos.org>
Date: Fri, 16 Jul 2021 09:37:45 +0200
Subject: [PATCH] Kernel: Support specifying a 64-bit KERNEL_BASE address

The kernel doesn't currently boot when using an address other than
0xc0000000 because the page tables aren't set up properly for that
but this at least lets us build the kernel.
---
 Kernel/Arch/x86/common/Boot/boot.S     | 38 ++++++++++++++++++++------
 Kernel/Arch/x86/x86_64/Boot/ap_setup.S |  6 +++-
 2 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/Kernel/Arch/x86/common/Boot/boot.S b/Kernel/Arch/x86/common/Boot/boot.S
index 2eadc29e26..28d3c348d6 100644
--- a/Kernel/Arch/x86/common/Boot/boot.S
+++ b/Kernel/Arch/x86/common/Boot/boot.S
@@ -519,20 +519,32 @@ pae_supported:
     movl %eax, %cr0
 
     /* set up stack */
-    mov $stack_top, %esp
+    mov $(stack_top - KERNEL_BASE), %esp
     and $-16, %esp
 
-    /* jump into C++ land */
-    addl $KERNEL_BASE, %ebx
-    movl %ebx, multiboot_info_ptr
-
 #if ARCH(X86_64)
     /* Now we are in 32-bit compatibility mode, We still need to load a 64-bit GDT */
-    lgdt gdt64ptr
-    ljmpl $code64_sel, $1f
+    mov $(gdt64ptr - KERNEL_BASE), %eax
+    lgdt (%eax)
+    ljmpl $code64_sel, $(1f - KERNEL_BASE)
 
 .code64
 1:
+    movl %ebx, %ebx
+    movabs $KERNEL_BASE, %rax
+    addq %rax, %rbx
+    movabs $multiboot_info_ptr, %rax
+    movq %rbx, (%rax)
+
+    movabs $gdt64ptr, %rax
+    lgdt (%rax)
+    movabs $1f, %rax
+    jmp *%rax
+
+1:
+    movabs $KERNEL_BASE, %rax
+    addq %rax, %rsp
+
     mov $0, %ax
     mov %ax, %ss
     mov %ax, %ds
@@ -542,11 +554,17 @@ pae_supported:
 
     mov %cr3, %rax
     mov %rax, %cr3
+
 #else
+    addl $KERNEL_BASE, %ebx
+    movl %ebx, multiboot_info_ptr
+
     /* jmp to an address above the 3GB mark */
-    movl $1f,%eax
+    movl $1f, %eax
     jmp *%eax
 1:
+    add $KERNEL_BASE, %esp
+
     movl %cr3, %eax
     movl %eax, %cr3
 #endif
@@ -561,10 +579,12 @@ pae_supported:
     addl $8, %edi
     loop 1b
 
-    call init
 #if ARCH(X86_64)
+    movabs $init, %rax
+    call *%rax
     add $4, %rsp
 #else
+    call init
     add $4, %esp
 #endif
 
diff --git a/Kernel/Arch/x86/x86_64/Boot/ap_setup.S b/Kernel/Arch/x86/x86_64/Boot/ap_setup.S
index b43f216a92..714642dd79 100644
--- a/Kernel/Arch/x86/x86_64/Boot/ap_setup.S
+++ b/Kernel/Arch/x86/x86_64/Boot/ap_setup.S
@@ -92,12 +92,16 @@ apic_ap_start32:
     movl %eax, %cr0
 
     /* load the temporary 64-bit gdt from boot that points above 3GB */
-    lgdt gdt64ptr
+    mov $(gdt64ptr - KERNEL_BASE), %eax
+    lgdt (%eax)
 
     /* jump above 3GB into our identity mapped area now */
     ljmpl $code64_sel, $(apic_ap_start64 - apic_ap_start + 0xc0008000)
 .code64
 apic_ap_start64:
+    movabs $gdt64ptr, %rax
+    lgdt (%rax)
+
     mov $0, %ax
     mov %ax, %ss
     mov %ax, %ds