[prefix] Use flat real mode for access to high memory

Use flat real mode rather than 16-bit protected mode for access to
high memory during installation.  This simplifies the code by reducing
the number of CPU modes we need to think about, and also increases the
amount of code in common between the normal and (somewhat
hypothetical) KEEP_IT_REAL methods of operation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/prefix/libprefix.S

 	.size	print_kill_line, . - print_kill_line
 
 /****************************************************************************
- * pm_call (real-mode near call)
+ * flatten_real_mode
  *
- * Call routine in 16-bit protected mode for access to extended memory
+ * Set up 4GB segment limits
  *
  * Parameters:
- *   %ax : address of routine to call in 16-bit protected mode
+ *   none
  * Returns:
  *   none
  * Corrupts:
- *   %ax
- *
- * The specified routine is called in 16-bit protected mode, with:
- *
- *   %cs : 16-bit code segment with base matching real-mode %cs
- *   %ss : 16-bit data segment with base matching real-mode %ss
- *   %ds,%es,%fs,%gs : 32-bit data segment with zero base and 4GB limit
- *
+ *   none
  ****************************************************************************
  */
-
 #ifndef KEEP_IT_REAL
 
 	/* GDT for protected-mode calls */
 	.section ".prefix.lib", "awx", @progbits
 	.align 16
-pm_call_vars:
-gdt:
-gdt_limit:		.word gdt_length - 1
-gdt_base:		.long 0
+flatten_vars:
+flatten_gdt:
+flatten_gdt_limit:	.word flatten_gdt_length - 1
+flatten_gdt_base:	.long 0
 			.word 0 /* padding */
-pm_cs:		/* 16-bit protected-mode flat code segment */
-	.equ    PM_CS, pm_cs - gdt
+flatten_cs:	/* 16-bit protected-mode flat code segment */
+	.equ    FLAT_CS, flatten_cs - flatten_gdt
 	.word   0xffff, 0
 	.byte   0, 0x9b, 0x8f, 0
-pm_ss:		/* 16-bit protected-mode flat stack segment */
-	.equ    PM_SS, pm_ss - gdt
+flatten_ss:	/* 16-bit protected-mode flat stack segment */
+	.equ    FLAT_SS, flatten_ss - flatten_gdt
 	.word   0xffff, 0
 	.byte   0, 0x93, 0x8f, 0
-pm_ds:		/* 32-bit protected-mode flat data segment */
-	.equ    PM_DS, pm_ds - gdt
-	.word   0xffff, 0
-	.byte   0, 0x93, 0xcf, 0
-gdt_end:
-	.equ	gdt_length, . - gdt
-	.size	gdt, . - gdt
+flatten_gdt_end:
+	.equ	flatten_gdt_length, . - flatten_gdt
+	.size	flatten_gdt, . - flatten_gdt
 
 	.section ".prefix.lib", "awx", @progbits
 	.align 16
-pm_saved_gdt:	
+flatten_saved_gdt:
 	.long	0, 0
-	.size	pm_saved_gdt, . - pm_saved_gdt
+	.size	flatten_saved_gdt, . - flatten_saved_gdt
 
-	.equ	pm_call_vars_size, . - pm_call_vars
-#define PM_CALL_VAR(x) ( -pm_call_vars_size + ( (x) - pm_call_vars ) )
+	.equ	flatten_vars_size, . - flatten_vars
+#define FLATTEN_VAR(x) ( -flatten_vars_size + ( (x) - flatten_vars ) )
 
 	.section ".prefix.lib", "awx", @progbits
 	.code16
-pm_call:
-	/* Preserve registers, flags, and RM return point */
+flatten_real_mode:
+	/* Preserve registers and flags, allocate local variable block */
 	pushw	%bp
 	movw	%sp, %bp
-	subw	$pm_call_vars_size, %sp
+	subw	$flatten_vars_size, %sp
 	andw	$0xfff0, %sp
 	pushfl
+	pushl	%eax
+	pushl	%edi
+	pushw	%si
+	pushw	%cx
 	pushw	%gs
 	pushw	%fs
 	pushw	%es
 	pushw	%ds
 	pushw	%ss
-	pushw	%cs
-	pushw	$99f
 
-	/* Set up local variable block, and preserve GDT */
-	pushw	%cx
-	pushw	%si
-	pushw	%di
+	/* Fill local variable block and preserve GDT */
 	pushw	%ss
 	popw	%es
-	movw	$pm_call_vars, %si
-	leaw	PM_CALL_VAR(pm_call_vars)(%bp), %di
-	movw	$pm_call_vars_size, %cx
+	movw	$flatten_vars, %si
+	leaw	FLATTEN_VAR(flatten_vars)(%bp), %di
+	movw	$flatten_vars_size, %cx
 	cs rep movsb
-	popw	%di
-	popw	%si
-	popw	%cx
-	sgdt	PM_CALL_VAR(pm_saved_gdt)(%bp)
+	sgdt	FLATTEN_VAR(flatten_saved_gdt)(%bp)
 
 	/* Set up GDT bases */
-	pushl	%eax
-	pushl	%edi
 	xorl	%eax, %eax
 	movw	%ss, %ax
 	shll	$4, %eax
 	movzwl	%bp, %edi
-	addr32 leal PM_CALL_VAR(gdt)(%eax, %edi), %eax
-	movl	%eax, PM_CALL_VAR(gdt_base)(%bp)
+	addr32 leal FLATTEN_VAR(flatten_gdt)(%eax, %edi), %eax
+	movl	%eax, FLATTEN_VAR(flatten_gdt_base)(%bp)
 	movw	%cs, %ax
-	movw	$PM_CALL_VAR(pm_cs), %di
+	movw	$FLATTEN_VAR(flatten_cs), %di
 	call	set_seg_base
 	movw	%ss, %ax
-	movw	$PM_CALL_VAR(pm_ss), %di
+	movw	$FLATTEN_VAR(flatten_ss), %di
 	call	set_seg_base
-	popl	%edi
-	popl	%eax
 
-	/* Switch CPU to protected mode and load up segment registers */
-	pushl	%eax
+	/* Switch temporarily to protected mode and set segment registers */
+	pushw	%cs
+	pushw	$2f
 	cli
-	data32 lgdt PM_CALL_VAR(gdt)(%bp)
+	data32 lgdt FLATTEN_VAR(flatten_gdt)(%bp)
 	movl	%cr0, %eax
 	orb	$CR0_PE, %al
 	movl	%eax, %cr0
-	ljmp	$PM_CS, $1f
-1:	movw	$PM_SS, %ax
+	ljmp	$FLAT_CS, $1f
+1:	movw	$FLAT_SS, %ax
 	movw	%ax, %ss
-	movw	$PM_DS, %ax
-	movw	%ax, %ds
-	movw	%ax, %es
-	movw	%ax, %fs
-	movw	%ax, %gs
-	popl	%eax
-
-	/* Call PM routine */
-	call	*%ax
-
-	/* Set real-mode segment limits on %ds, %es, %fs and %gs */
-	movw	%ss, %ax
 	movw	%ax, %ds
 	movw	%ax, %es
 	movw	%ax, %fs
 	movw	%ax, %gs
-
-	/* Return CPU to real mode */
 	movl	%cr0, %eax
 	andb	$0!CR0_PE, %al
 	movl	%eax, %cr0
+	lret
+2:	/* lret will ljmp to here */
 
-	/* Restore registers and flags */
-	lret	/* will ljmp to 99f */
-99:	popw	%ss
+	/* Restore GDT, registers and flags */
+	data32 lgdt FLATTEN_VAR(flatten_saved_gdt)(%bp)
+	popw	%ss
 	popw	%ds
 	popw	%es
 	popw	%fs
 	popw	%gs
-	data32 lgdt PM_CALL_VAR(pm_saved_gdt)(%bp)
+	popw	%cx
+	popw	%si
+	popl	%edi
+	popl	%eax
 	popfl
 	movw	%bp, %sp
 	popw	%bp
 	ret
-	.size pm_call, . - pm_call
+	.size flatten_real_mode, . - flatten_real_mode
 
 set_seg_base:
 	rolw	$4, %ax
 #endif /* KEEP_IT_REAL */
 
 /****************************************************************************
- * copy_bytes (real-mode or 16-bit protected-mode near call)
+ * copy_bytes
  *
  * Copy bytes
  *
  *   None
  ****************************************************************************
  */
+#if ! COMPRESS
 	.section ".prefix.lib", "awx", @progbits
 	.code16
 copy_bytes:
 	popl %ecx
 	ret
 	.size copy_bytes, . - copy_bytes
+#endif /* COMPRESS */
 
 /****************************************************************************
- * install_block (real-mode near call)
+ * install_block
  *
  * Install block to specified address
  *
 	.section ".prefix.lib", "awx", @progbits
 	.code16
 install_block:
-	
-#ifdef KEEP_IT_REAL
-
 	/* Preserve registers */
 	pushw	%ds
 	pushw	%es
 	pushl	%ecx
 	pushl	%edi
 	
-	/* Convert %esi and %edi to segment registers */
+	/* Convert %esi and %edi to %ds:esi and %es:edi */
 	shrl	$4, %esi
 	movw	%si, %ds
 	xorw	%si, %si
+	shll	$4, %esi
 	shrl	$4, %edi
 	movw	%di, %es
 	xorw	%di, %di
+	shll	$4, %edi
 
-#else /* KEEP_IT_REAL */
-
-	/* Call self in protected mode */
-	pushw	%ax
-	movw	$1f, %ax
-	call	pm_call
-	popw	%ax
-	ret
-1:
-	/* Preserve registers */
-	pushl	%ecx
-	pushl	%edi
-	
-#endif /* KEEP_IT_REAL */
-
-	
 #if COMPRESS
 	/* Decompress source to destination */
 	call	decompress16
 	addl	$0xf, %esi
 	andl	$~0xf, %esi
 
-
-#ifdef KEEP_IT_REAL
-
 	/* Convert %ds:esi back to a physical address */
-	movzwl	%ds, %cx
+	xorl	%ecx, %ecx
+	movw	%ds, %cx
 	shll	$4, %ecx
 	addl	%ecx, %esi
 
-	/* Restore registers */
+	/* Restore registers and return */
 	popl	%edi
 	popl	%ecx
 	popw	%es
 	popw	%ds
-
-#else /* KEEP_IT_REAL */
-
-	/* Restore registers */
-	popl	%edi
-	popl	%ecx
-
-#endif
-
 	ret
 	.size install_block, . - install_block
-	
+
 /****************************************************************************
- * alloc_basemem (real-mode near call)
+ * alloc_basemem
  *
  * Allocate space for .text16 and .data16 from top of base memory.
  * Memory is allocated using the BIOS free base memory counter at
 	.size alloc_basemem, . - alloc_basemem
 
 /****************************************************************************
- * free_basemem (real-mode near call)
+ * free_basemem
  *
  * Free space allocated with alloc_basemem.
  *
 	.size	hooked_bios_interrupts, . - hooked_bios_interrupts
 
 /****************************************************************************
- * install (real-mode near call)
+ * install
  *
  * Install all text and data segments.
  *
 	.size install, . - install
 
 /****************************************************************************
- * install_prealloc (real-mode near call)
+ * install_prealloc
  *
  * Install all text and data segments.
  *
 	/* Sanity: clear the direction flag asap */
 	cld
 
+#ifndef KEEP_IT_REAL
+	/* Flatten real mode */
+	call	flatten_real_mode
+#endif
+
 	/* Calculate physical address of payload (i.e. first source) */
 	testl	%esi, %esi
 	jnz	1f
 
 	/* Copy code to new location */
 	pushl	%edi
-	pushw	%ax
-	movw	$copy_bytes, %ax
-	call	pm_call
-	popw	%ax
+	xorw	%ax, %ax
+	movw	%ax, %es
+	es rep addr32 movsb
 	popl	%edi
 
 	/* Initialise librm at new location */
 	lcall	*init_librm_vector
-
 #endif
+
 	/* Restore registers */
 	popw	%es
 	popw	%ds
 	ret
 	.size install_prealloc, . - install_prealloc
 
-	/* Vectors for far calls to .text16 functions */
+	/* Vectors for far calls to .text16 functions.  Must be in
+	 * .data16, since .prefix may not be writable.
+	 */
 	.section ".data16", "aw", @progbits
 #ifdef KEEP_IT_REAL
 init_libkir_vector:
 #endif
 
 /****************************************************************************
- * uninstall (real-mode near call)
+ * uninstall
  *
  * Uninstall all text and data segments.
  *