[lkrnprefix] Function as a bzImage kernel

The .lkrn prefix currently provides a zImage kernel with unused setup
sectors and the whole iPXE binary placed within the "protected mode
kernel" portion of the zImage.

The work carried out years ago to create the .mrom format provides a
mechanism allowing the iPXE binary to be split into a small real-mode
header and a larger payload.  This neatly matches the way that a
bzImage is loaded: the "setup sectors" can contain the header and the
"protected mode kernel" can contain the payload.

This removes the size restrictions on an iPXE .lkrn image (and hence
on derived image formats such as .iso).

Also remove obsolete copyright information, since none of the original
code or functionality now remains.

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

src/arch/i386/prefix/lkrnprefix.S

@@ -1,123 +1,59 @@
-/*
-	Copyright (C) 2000, Entity Cyber, Inc.
-
-	Authors: Gary Byers (gb@thinguin.org)
-		 Marty Connor (mdc@thinguin.org)
-
-	This software may be used and distributed according to the terms
-	of the GNU Public License (GPL), incorporated herein by reference.
-
-	Description:	
-
-	This is just a little bit of code and data that can get prepended
-	to a ROM image in order to allow bootloaders to load the result
-	as if it were a Linux kernel image.
-
-	A real Linux kernel image consists of a one-sector boot loader
-	(to load the image from a floppy disk), followed a few sectors
-	of setup code, followed by the kernel code itself.  There's
-	a table in the first sector (starting at offset 497) that indicates
-	how many sectors of setup code follow the first sector and which
-	contains some other parameters that aren't interesting in this
-	case.
-
-	When a bootloader loads the sectors that comprise a kernel image,
-	it doesn't execute the code in the first sector (since that code
-	would try to load the image from a floppy disk.)  The code in the
-	first sector below doesn't expect to get executed (and prints an
-	error message if it ever -is- executed.)
-
-	We don't require much in the way of setup code.  Historically, the
-	Linux kernel required at least 4 sectors of setup code.
-	Therefore, at least 4 sectors must be present even though we don't
-	use them.
-
-*/
-
 FILE_LICENCE ( GPL_ANY )
 
-#define	SETUPSECS 4		/* Minimal nr of setup-sectors */
-#define PREFIXSIZE ((SETUPSECS+1)*512)
-#define PREFIXPGH (PREFIXSIZE / 16 )
-#define	BOOTSEG  0x07C0		/* original address of boot-sector */
-#define	INITSEG  0x9000		/* we move boot here - out of the way */
-#define	SETUPSEG 0x9020		/* setup starts here */
-#define SYSSEG   0x1000		/* system loaded at 0x10000 (65536). */
+#define BZI_RM_SEGMENT 0x1000
+#define BZI_LOAD_HIGH_ADDR 0x100000
 
 	.text
-	.code16
 	.arch i386
-	.org	0
+	.code16
 	.section ".prefix", "ax", @progbits
 	.globl	_lkrn_start
 _lkrn_start:
-/* 
-	This is a minimal boot sector.	If anyone tries to execute it (e.g., if
-	a .lilo file is dd'ed to a floppy), print an error message. 
-*/
-
-bootsector: 
-	jmp	$BOOTSEG, $1f	/* reload cs:ip to match relocation addr */
-1:
-	movw	$0x2000, %di		/*  0x2000 is arbitrary value >= length
-					    of bootsect + room for stack */
-
-	movw	$BOOTSEG, %ax
-	movw	%ax,%ds
-	movw	%ax,%es
-
-	cli
-	movw	%ax, %ss		/* put stack at BOOTSEG:0x2000. */
-	movw	%di,%sp
-	sti
-
-	movw	$why_end-why, %cx
-	movw	$why, %si
-
-	movw	$0x0007, %bx		/* page 0, attribute 7 (normal) */
-	movb	$0x0e, %ah		/* write char, tty mode */
-prloop: 
-	lodsb
-	int	$0x10
-	loop	prloop
-freeze: jmp	freeze
-
-why:	.ascii	"This image cannot be loaded from a floppy disk.\r\n"
-why_end: 
 
+/*****************************************************************************
+ *
+ * Kernel header
+ *
+ * We place our prefix (i.e. our .prefix and .text16.early sections)
+ * within the bzImage real-mode portion which gets loaded at
+ * 1000:0000, and our payload (i.e. everything else) within the
+ * bzImage protected-mode portion which gets loaded at 0x100000
+ * upwards.
+ *
+ */
 
-/*
-	The following header is documented in the Linux source code at
-	Documentation/x86/boot.txt
-*/
-	.org	497
-setup_sects: 
-	.byte	SETUPSECS
-root_flags: 
+	.org	0x1f1
+setup_sects:
+	.byte	-1 /* Allow for initial "boot sector" */
+	.section ".zinfo.fixup", "a", @progbits	/* Compressor fixups */
+	.ascii	"ADHL"
+	.long	setup_sects
+	.long	512
+	.long	0
+	.previous
+root_flags:
 	.word	0
-syssize: 
-	.long	-PREFIXPGH
-
+syssize:
+	.long	0
 	.section ".zinfo.fixup", "a", @progbits	/* Compressor fixups */
-	.ascii	"ADDL"
+	.ascii	"ADPL"
 	.long	syssize
 	.long	16
 	.long	0
 	.previous
-	
-ram_size: 
+ram_size:
 	.word	0
-vid_mode: 
+vid_mode:
 	.word	0
-root_dev: 
+root_dev:
 	.word	0
-boot_flag: 
-	.word	0xAA55
+boot_flag:
+	.word	0xaa55
 jump:
 	/* Manually specify a two-byte jmp instruction here rather
-	 * than leaving it up to the assembler. */
-	.byte	0xeb
-	.byte	setup_code - header
+	 * than leaving it up to the assembler.
+	 */
+	.byte	0xeb, ( setup - header )
 header:
 	.byte	'H', 'd', 'r', 'S'
 version:
@@ -125,13 +61,13 @@ version:
 realmode_swtch:
 	.long	0
 start_sys:
-	.word	0
+	.word	BZI_RM_SEGMENT
 kernel_version:
 	.word	version_string - 0x200
 type_of_loader:
 	.byte	0
 loadflags:
-	.byte	0
+	.byte	0x01 /* LOADED_HIGH */
 setup_move_size:
 	.word	0
 code32_start:
@@ -144,21 +80,22 @@ bootsect_kludge:
 	.long	0
 heap_end_ptr:
 	.word	0
-pad1:
-	.word	0
+ext_loader_ver:
+	.byte	0
+ext_loader_type:
+	.byte	0
 cmd_line_ptr:
 	.long	0
 initrd_addr_max:
-	/* We don't use an initrd but some bootloaders (e.g. SYSLINUX) have
-	 * been known to require this field.  Set the value to 2 GB.  This
-	 * value is also used by the Linux kernel. */
-	.long	0x7fffffff
+	.long	0xffffffff
 kernel_alignment:
 	.long	0
 relocatable_kernel:
 	.byte	0
-pad2:
-	.byte	0, 0, 0
+min_alignment:
+	.byte	0
+xloadflags:
+	.word	0
 cmdline_size:
 	.long	0x7ff
 hardware_subarch:
@@ -169,28 +106,16 @@ hardware_subarch_data:
 version_string:
 	.asciz	VERSION
 
-/*
-	We don't need to do too much setup.
-
-	This code gets loaded at SETUPSEG:0.  It wants to start
-	executing the image that's loaded at SYSSEG:0 and
-	whose entry point is SYSSEG:0.
-*/
-setup_code:
-	/* We expect to be contiguous in memory once loaded.  The Linux image
-	 * boot process requires that setup code is loaded separately from
-	 * "non-real code".  Since we don't need any information that's left
-	 * in the prefix, it doesn't matter: we just have to ensure that
-	 * %cs:0000 is where the start of the image *would* be.
-	 */
-	ljmp	$(SYSSEG-(PREFIXSIZE/16)), $run_ipxe
-
-
-	.org	PREFIXSIZE
-/*
-	We're now at the beginning of the kernel proper.
+/*****************************************************************************
+ *
+ * Setup code
+ *
  */
-run_ipxe:
+
+setup:
+	/* Fix up code segment */
+	ljmp	$BZI_RM_SEGMENT, $1f
+1:
 	/* Set up stack just below 0x7c00 and clear direction flag */
 	xorw	%ax, %ax
 	movw	%ax, %ss
@@ -198,7 +123,7 @@ run_ipxe:
 	cld
 
 	/* Retrieve command-line pointer */
-	movl	%ds:cmd_line_ptr, %edx
+	movl	cmd_line_ptr, %edx
 	testl	%edx, %edx
 	jz	no_cmd_line
 
@@ -240,7 +165,6 @@ no_cmd_line:
 	jnz	1f
 	orl	$0xffffffff, %ebp /* Allow arbitrary relocation if no initrd */
 1:
-
 	/* Install iPXE */
 	call	alloc_basemem
 	xorl	%esi, %esi
@@ -282,3 +206,30 @@ no_cmd_line:
 
 	/* Boot next device */
 	int $0x18
+
+/*****************************************************************************
+ *
+ * Open payload (called by libprefix)
+ *
+ * Parameters:
+ *   %ds:0000 : Prefix
+ *   %esi : Buffer for copy of image source (or zero if no buffer available)
+ *   %ecx : Expected offset within buffer of first payload block
+ * Returns:
+ *   %esi : Valid image source address (buffered or unbuffered)
+ *   %ecx : Actual offset within buffer of first payload block
+ *   CF set on error
+ */
+
+	.section ".text16.early", "awx", @progbits
+	.globl	open_payload
+open_payload:
+
+	/* Our payload will always end up at BZI_LOAD_HIGH_ADDR */
+	movl	$BZI_LOAD_HIGH_ADDR, %esi
+	xorl	%ecx, %ecx
+	lret
+
+	/* Payload must be aligned to a whole number of setup sectors */
+	.globl	_payload_align
+	.equ	_payload_align, 512