[romprefix] Add .mrom format, allowing loading of large ROMs

Add an infrastructure allowing the prefix to provide an open_payload()
method for obtaining out-of-band access to the whole iPXE image.  Add
a mechanism within this infrastructure that allows raw access to the
expansion ROM BAR by temporarily borrowing an address from a suitable
memory BAR on the same PCI card.

For cards that have a memory BAR that is at least as large as their
expansion ROM BAR, this allows large iPXE ROMs to be supported even on
systems where PMM fails, or where option ROM space pressure makes it
impossible to use PMM shrinking.  The BIOS sees only a stub ROM of
approximately 3kB in size; the remainder (which can be well over 64kB)
is loaded only at the time iPXE is invoked.

As a nice side-effect, an iPXE .mrom image will continue to work even
if its PMM-allocated areas are overwritten between initialisation and
invocation.

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

src/arch/i386/prefix/romprefix.S

@@ -32,6 +32,16 @@ FILE_LICENCE ( GPL2_OR_LATER )
  */
 #define ROM_BANNER_TIMEOUT ( 2 * ( 18 * BANNER_TIMEOUT ) / 10 )
 
+/* Allow payload to be excluded from ROM size
+ */
+#if ROMPREFIX_EXCLUDE_PAYLOAD
+#define	ZINFO_TYPE_ADxB "ADHB"
+#define	ZINFO_TYPE_ADxW "ADHW"
+#else
+#define	ZINFO_TYPE_ADxB "ADDB"
+#define	ZINFO_TYPE_ADxW "ADDW"
+#endif
+
 	.text
 	.code16
 	.arch i386
@@ -53,7 +63,7 @@ checksum:
 	.size romheader, . - romheader
 
 	.section ".zinfo.fixup", "a", @progbits	/* Compressor fixups */
-	.ascii	"ADDB"
+	.ascii	ZINFO_TYPE_ADxB
 	.long	romheader_size
 	.long	512
 	.long	0
@@ -83,11 +93,11 @@ pciheader_runtime_length:
 	.size pciheader, . - pciheader
 
 	.section ".zinfo.fixup", "a", @progbits	/* Compressor fixups */
-	.ascii	"ADDW"
+	.ascii	ZINFO_TYPE_ADxW
 	.long	pciheader_image_length
 	.long	512
 	.long	0
-	.ascii	"ADDW"
+	.ascii	ZINFO_TYPE_ADxW
 	.long	pciheader_runtime_length
 	.long	512
 	.long	0
@@ -179,6 +189,9 @@ init:
 	movw	%bx, %gs
 	movw	%di, %bx
 
+	/* Store PCI bus:dev.fn address */
+	movw	%ax, init_pci_busdevfn
+
 	/* Print message as early as possible */
 	movw	$init_message, %si
 	xorw	%di, %di
@@ -320,8 +333,7 @@ pmm_scan:
 	/* We have PMM and so a 1kB stack: preserve whole registers */
 	pushal
 	/* Allocate image source PMM block */
-	movzbl	romheader_size, %ecx
-	shll	$5, %ecx
+	movzwl	image_source_len_pgh, %ecx
 	movl	$PMM_HANDLE_BASE_IMAGE_SOURCE, %ebx
 	movw	$get_pmm_image_source, %bp
 	call	get_pmm
@@ -552,6 +564,13 @@ init_message_done:
 	.asciz	"\n\n"
 	.size	init_message_done, . - init_message_done
 
+/* PCI bus:dev.fn
+ *
+ */
+init_pci_busdevfn:
+	.word	0xffff
+	.size	init_pci_busdevfn, . - init_pci_busdevfn
+
 /* Image source area
  *
  * May be either zero (indicating to use option ROM space as source),
@@ -562,6 +581,19 @@ image_source:
 	.long	0
 	.size	image_source, . - image_source
 
+/* Image source area length (in paragraphs)
+ *
+ */
+image_source_len_pgh:
+	.word	0
+	.size	image_source_len_pgh, . - image_source_len_pgh
+	.section ".zinfo.fixup", "a", @progbits	/* Compressor fixups */
+	.ascii	"ADDW"
+	.long	image_source_len_pgh
+	.long	16
+	.long	0
+	.previous
+
 /* Shrunk ROM size (in 512-byte sectors)
  *
  */