mm: provide high memory kernel mapping in asm code.

we use the first pml4 entry (+one pdp table) to map phy 0~512G
to virt 0~512G for init code. This doesn't change.

For the kernel to work in higher half memory, we also need to create
mapping for it. We take the 256th entry of pml4 entry (hence one
additional pdp table).

Entry 0~63 are mapped to to the physical memory
(with offset 0xffff_8000_0000_0000)
Entry 64~127 are not used
Entry 128~191 are mapped to the kernel image (text and code)
(with offset 0xffff_8020_0000_0000)

details in docs/mem_layout.txt

Author: shrik3

boot/startup-x86_64.s

@@ -5,7 +5,11 @@
 STACKSIZE: equ 65536
 
 ; 512 GB maximum RAM size for page table
-; DON'T MODIFY THIS UNLESS YOU UPDATE THE setup_paging accordingly
+; DON'T MODIFY THIS UNLESS YOU UPDATE THE setup_paging accordingly.
+; IMPORTANT! regardless of the initial mapping size, we limit the phy memory to
+; 64GB in the actual paging, so that all kernel VMAs could fit into one pml4
+; entry and one pdp (level 3) table. See docs/mem_layout.txt
+
 MAX_MEM: equ 512
 
 ; be careful with the extern and exported symbols when mapping a higher-half
@@ -23,8 +27,8 @@ MAX_MEM: equ 512
 [GLOBAL mb_info_addr]
 ; functions from other parts of rustubs
 [EXTERN _entry]
-[EXTERN ___BSS_START__]
-[EXTERN ___BSS_END__]
+[EXTERN ___BSS_PM_START__]
+[EXTERN ___BSS_PM_END__]
 
 [SECTION .text]
 
@@ -68,16 +72,16 @@ setup_paging:
 	; zero out the initial page tables (2 pages in total)
 	mov    edi, pml4
 clear_pt:
-	mov    byte [edi], 0
-	inc    edi
+	mov    dword [edi], 0
+	add    edi, 4
 	cmp    edi, pt_end
-	jne    clear_pt
+	jl     clear_pt
 
 	; Provisional identical page mapping, using 1G huge page, therefore only 2
 	; table levels needed. see docs/x86_paging.txt
 
 	; PML4 (Page Map Level 4 / 1st level)
-	mov    eax, pdp
+	mov    eax, pdp0
 	or	   eax, 0xf
 	mov    dword [pml4+0], eax
 	mov    dword [pml4+4], 0
@@ -89,8 +93,8 @@ fill_tables2:
 	; fill one single PDP table, with 1G pages, 512 PDPE maps to 512 GB
 	cmp    ecx, MAX_MEM
 	je	   fill_tables2_done
-	mov    dword [pdp + 8*ecx + 0], eax ; low bytes
-	mov    dword [pdp + 8*ecx + 4], ebx ; high bytes
+	mov    dword [pdp0 + 8*ecx + 0], eax ; low bytes
+	mov    dword [pdp0 + 8*ecx + 4], ebx ; high bytes
 	add    eax, 0x40000000				; 1G per page
 	adc    ebx, 0			  ; overflow? -> increment higher-order half of the address
 	inc    ecx
@@ -115,32 +119,74 @@ activate_long_mode:
 	; jump to 64-bit code segment -> full activation of Long Mode
 	jmp    2 * 0x8 : longmode_start
 
+
+; =====================================================================
+; MUST NOT USE ANY 64 BIT SYMBOLS BEFORE THIS POINT!
+; may include:
+; - symbols defined in 64 bit code below, if mapped to higher memory (VA)
+; - all symbols exported from rust code or linker script
+; =====================================================================
+
 [BITS 64]
-	;	system start, part 2 (in 64-bit Long Mode)
 longmode_start:
-	mov    rdi, ___BSS_START__
+	; now we set the pagetables for higher half memory
+	; since we have Provisional paging now, why not using 64bit code?
+	mov    eax, pdp1
+	or 	   eax, 0xf
+	mov    dword [pml4+256], eax
+	mov    dword [pml4+256+4], 0
+	; PDPE flags, see above
+
+	; entry 0~63 is an identical mapping with offset 0x8000_0000_0000
+	; clear the BSS section before going to rust code
+	; TODO this should not be executable
+	mov    rax, 0x0
+	or     rax, 0x87
+	mov    rdi, 0
+fill_kvma1:
+	mov    qword [pdp1 + 8*rdi], rax
+	inc    rdi
+	add    rax, 0x40000000
+	cmp    rdi, 64
+	jne    fill_kvma1
+
+	; entry 64~127 is a hole (also some sort of protection)
+	; entry 128~191 are mapping of the kernel image itself
+	mov    rax, 0x0
+	or     rax, 0x87
+	mov    rdi, 128
+fill_kvma2:
+	mov    qword [pdp1 + 8*rdi], rax
+	inc    rdi
+	add    rax, 0x40000000
+	cmp    rdi, 192
+	jne    fill_kvma2
+	; done :-)
+
+	; clear BSS section for the rust code.
+	mov    rdi, ___BSS_PM_START__
 clear_bss:
 	mov    byte [rdi], 0
 	inc    rdi
-	cmp    rdi, ___BSS_END__
+	cmp    rdi, ___BSS_PM_END__
 	jne    clear_bss
-	fninit		   ; activate FPU
-
-init_sse:
-	; init SSE
-	; NOTE: must NOT use sse target features for rust compiler, if sse not enabled here.
-	;mov rax, cr0
-	;and rax, ~(1 << 2)	;clear coprocessor emulation CR0.EM
-	;or rax, 1 << 1		;set coprocessor monitoring  CR0.MP
-	;mov cr0, rax
-	;mov rax, cr4
-	;or rax, 3 << 9		;set CR4.OSFXSR and CR4.OSXMMEXCPT at the same time
-	;mov cr4, rax
-
-	call   _entry  ; call the OS kernel's rust part.
-	cli			   ; Usually we should not get here.
+	; enable FPU
+	fninit
+
+	; NOTE: must NOT use sse target features for rust compiler, if sse not
+	; enabled here.
+
+	; finally go to the rust code!
+	call   _entry
+	; should not reach below
+	cli
 	hlt
 
+; =====================================================================
+; data sections they should all have VAs identical to their PAs
+; so we map these symbols differently than those generated by rust code
+; =====================================================================
+
 [SECTION .data]
 
 gdt:
@@ -195,12 +241,21 @@ init_stack:
 
 [SECTION .global_pagetable]
 
-
+; create initial page tables wrt. the memory layout
+; we use entry 0 and 256 of the PML4 table
+; the whole of the first first pdp table (512G)
+; and entry 0, 2 of the second pdp table
+; all pages here are 1 GiB huge pages
 pml4:
 	resb   4096
 	alignb 4096
-
-pdp:
+; the first PDP covers the lower 512GiB memory
+pdp0:
+	resb   4096
+	alignb 4096
+; pdp1 does the same but in higher half memory with offset
+; 0xffff_8000_0000_0000, i.e. the 256th entry of pml4
+pdp1:
 	resb   4096
 	alignb 4096
 pt_end:

defs/x86_64-linker.ld

@@ -30,7 +30,7 @@ SECTIONS
 		header_end = .;
 	}
 
-	PROVIDE (___KERNEL_START__ = .);
+	PROVIDE (___KERNEL_PM_START__ = .);
 	.text :
 	{
 		*(".text")
@@ -71,10 +71,10 @@ SECTIONS
 
 	.bss :
 	{
-		PROVIDE (___BSS_START__ = .);
+		PROVIDE (___BSS_PM_START__ = .);
 		*(".bss")
 		*(".bss.*")
-		PROVIDE (___BSS_END__ = .);
+		PROVIDE (___BSS_PM_END__ = .);
 	}
 
 	/* global page table for 64-bit long mode */
@@ -95,5 +95,5 @@ SECTIONS
 		*("..global_free_page_stack")
 	}
 	. = ALIGN(4096);
-	PROVIDE (___KERNEL_END__ = .);
+	PROVIDE (___KERNEL_PM_END__ = .);
 }

src/defs.rs

@@ -1,25 +1,25 @@
 // exported symbols from asm/linker.
 // They are always unsafe.
 extern "C" {
-	fn ___KERNEL_START__();
-	fn ___KERNEL_END__();
-	fn ___BSS_START__();
-	fn ___BSS_END__();
+	fn ___KERNEL_PM_START__();
+	fn ___KERNEL_PM_END__();
+	fn ___BSS_PM_START__();
+	fn ___BSS_PM_END__();
 }
 
 pub fn pmap_kernel_start() -> u64 {
-	return ___KERNEL_START__ as u64;
+	return ___KERNEL_PM_START__ as u64;
 }
 
 pub fn pmap_kernel_end() -> u64 {
-	return ___KERNEL_END__ as u64;
+	return ___KERNEL_PM_END__ as u64;
 }
 pub fn pmap_bss_start() -> u64 {
-	return ___BSS_START__ as u64;
+	return ___BSS_PM_START__ as u64;
 }
 
 pub fn pmap_bss_end() -> u64 {
-	return ___BSS_END__ as u64;
+	return ___BSS_PM_END__ as u64;
 }
 
 pub fn roundup_4k(addr: u64) -> u64 {