Fix bug in GDT when using KVM acceleration
The bug was due to the DS segment being set to the 0th segment entry, causing a general protection fault.
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0d16e3e35a
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fd8b5bd57e
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@ -2,6 +2,8 @@
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GDT_MEMORY_AREA equ 0x7e00
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GDT_ENTRY_COUNT equ 3
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; Arguments (note: all arguments are 4-byte padded):
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; [Furthest from EBP]
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;
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@ -109,114 +111,6 @@ make_regular_data_segment_in_gdt:
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; Arguments (note: all arguments are 4-byte padded):
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; [Furthest from EBP]
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;
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; 4. uint32 privilege_level
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; - Only 0 to 3; 0 being kernel and 3 being userland.
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; Everything above 3 is an error.
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;
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; 3 uint32 num_pages
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; - Only 20 bits are usable. Everything above will
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; be cut off to be written to the segment descriptor.
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;
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; 2. ptr32 base_address
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;
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; 1. uint32 segment_index
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; Note:
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; - Zero is NOT usable and will throw an error.
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; the highest theoretical value for this is 8191,
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; but generally, one should stick to GDT_ENTRY_COUNT.
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;
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; [Nearest to EBP]
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; COMMENTED OUT DUE TO BEING BUGGY!
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; make_stack_segment_in_gdt:
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; .prolog:
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; push esi
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; add esp, 64
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; mov esi, esp
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; mov [esi + (64 - 4)], eax
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; mov [esi + (64 - 8)], ebx
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; mov [esi + (64 - 12)], ecx
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; mov [esi + (64 - 16)], edx
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; mov [esi + (64 - 20)], edi
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; .get_arguments:
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; mov eax, [ebp - 4] ; segment_index
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; mov ebx, [ebp - 8] ; base_address
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; mov ecx, [ebp - 12] ; num_pages
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; mov edx, [ebp - 16] ; privilege_level
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; mov [esi + 16], eax ; segment_index
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; mov [esi + 20], ebx ; base_address
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; mov [esi + 24], ecx ; num_pages
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; mov [esi + 28], edx ; privilege_level
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; .validate_arguments:
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; cmp eax, 8191
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; ja .segment_index_too_tall
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; cmp eax, 0
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; je .segment_index_is_zero
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; cmp edx, 3
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; ja .privilege_level_too_large
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; .calculate_work_address:
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; shl eax, 3 ; Entry offset from table start
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; add eax, GDT_MEMORY_AREA ; Entry address
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; mov [esi + 12], eax
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; .write_segment_limit:
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; mov [eax], cx
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; shr ecx, 16
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; and ecx, 0x0f
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; mov [eax + 6], cl
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; .write_base_address:
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; mov [eax + 2], bx
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; shr bx, 16
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; mov [eax + 4], bl
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; mov [eax + 7], bh
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; .set_default_flags:
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; or [eax + 6], word (0b1100 << 4)
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; .set_stack_specific_access_byte:
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; mov al, 0b1001
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; ; OR the privilege level into the access byte
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; mov dl, [ebp - 16]
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; shl dl, 1
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; and dl, 0b0110
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; or al, dl
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; shl al, 4 ; Move the 4 bits of general flags to their place
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; ; Stack specific part
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; or al, 0b0110 ; DATA | STACK | READ_WRITE | NOT_ACCESSED
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; mov ebx, [esi + 12]
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; mov [ebx + 5], al ; Write to the final destination
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; .epilog:
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; mov edi, [esi + (64 - 20)]
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; mov edx, [esi + (64 - 16)]
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; mov ecx, [esi + (64 - 12)]
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; mov ebx, [esi + (64 - 8)]
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; mov eax, [esi + (64 - 4)]
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; sub esp, 64
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; pop esi
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; ret
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; .segment_index_too_tall:
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; .segment_index_is_zero:
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; .privilege_level_too_large:
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; ; todo(logs): Write debug message.
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; cli
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; hlt
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; Arguments (note: all arguments are 4-byte padded):
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; [Furthest from EBP]
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;
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@ -323,6 +217,7 @@ make_code_segment_in_gdt:
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hlt
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; [Furthest to EBP]
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; 1) Return Address
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; [Nearest to EBP]
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@ -352,17 +247,6 @@ load_flat_gdt:
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mov esp, ebp
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pop ebp
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; COMMENTED OUT DUE TO BEING BUGGY
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; push ebp
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; mov ebp, esp
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; push dword 3 ; Segment Index
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; push dword LOWER_STACK_POINTER ; Start Offset
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; push dword 0xfffff ; Page Count
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; push dword 0 ; Required privilege level
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; call make_stack_segment_in_gdt
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; mov esp, ebp
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; pop ebp
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push ebp
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mov ebp, esp
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push dword 1 ; Segment Index
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@ -382,16 +266,12 @@ load_flat_gdt:
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mov es, bx
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mov fs, bx
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mov gs, bx
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; mov bx, (3 << 3)
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mov ss, bx
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mov ebx, cr0
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or ebx, 1
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mov cr0, ebx
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xor bx, bx
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mov ds, bx
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jmp dword 0x08:.epilog
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bits 32
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@ -402,10 +282,7 @@ bits 32
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mov ebx, [esi + (64 - 8)]
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mov eax, [esi + (64 - 4)]
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add esp, 64
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pop esi
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jmp stage2_true_entry
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; jmp after_gdt_loaded
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jmp dword stage2_true_entry
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align 16
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.gdtr:
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