[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/libprefix.S

@@ -479,6 +479,10 @@ install_prealloc:
 	cld			/* Sanity: clear the direction flag asap */
 	pushfw
 
+	/* Set up %ds for (read-only) access to .prefix */
+	pushw	%cs
+	popw	%ds
+
 	/* Copy decompression temporary area physical address to %ebp */
 	movl	%edi, %ebp
 
@@ -495,7 +499,6 @@ install_prealloc:
 	call	install_block		/* .text16.early */
 	popl	%esi
 
-	/* Open up access to payload */
 #ifndef KEEP_IT_REAL
 	/* Access high memory */
 	pushw	%cs
@@ -511,18 +514,39 @@ install_prealloc:
 2:	jmp	2b
 	.section ".prefix.data", "aw", @progbits
 a20_death_message:
-	.asciz	"Gate A20 stuck - cannot continue\n"
+	.asciz	"\nHigh memory inaccessible - cannot continue\n"
 	.size	a20_death_message, . - a20_death_message
 	.previous
 3:
 #endif
 
+	/* Open payload (which may not yet be in memory) */
+	pushw	%cs
+	pushw	$1f
+	pushw	%ax
+	pushw	$open_payload
+	lret
+1:	/* Die if we could not access the payload */
+	jnc	3f
+	xorw	%di, %di
+	movl	%esi, %eax
+	call	print_hex_dword
+	movw	$payload_death_message, %si
+	call	print_message
+2:	jmp	2b
+	.section ".prefix.data", "aw", @progbits
+payload_death_message:
+	.asciz	"\nPayload inaccessible - cannot continue\n"
+	.size	payload_death_message, . - payload_death_message
+	.previous
+3:
+
 	/* Calculate physical address of payload (i.e. first source) */
 	testl	%esi, %esi
 	jnz	1f
 	movw	%cs, %si
 	shll	$4, %esi
-1:	addl	%cs:payload_lma, %esi
+1:	addl	payload_lma, %esi
 
 	/* Install .text16.late and .data16 */
 	movl	$_text16_late_filesz, %ecx
@@ -588,9 +612,11 @@ a20_death_message:
 
 	/* Copy code to new location */
 	pushl	%edi
+	pushw	%ax
 	xorw	%ax, %ax
 	movw	%ax, %es
 	es rep addr32 movsb
+	popw	%ax
 	popl	%edi
 
 	/* Initialise librm at new location */
@@ -598,6 +624,10 @@ a20_death_message:
 skip_relocate:
 #endif
 
+	/* Close access to payload */
+	movw	%ax, (close_payload_vector+2)
+	lcall	*close_payload_vector
+
 	/* Restore registers */
 	popfw
 	popw	%es
@@ -625,6 +655,10 @@ prot_call_vector:
 	.word 0
 	.size prot_call_vector, . - prot_call_vector
 #endif
+close_payload_vector:
+	.word close_payload
+	.word 0
+	.size close_payload_vector, . - close_payload_vector
 
 	/* Payload address */
 	.section ".prefix.lib", "awx", @progbits
@@ -637,6 +671,17 @@ payload_lma:
 	.long	0
 	.previous
 
+	/* Dummy routines to open and close payload */
+	.section ".text16.early.data", "aw", @progbits
+	.weak	open_payload
+	.weak	close_payload
+open_payload:
+close_payload:
+	clc
+	lret
+	.size	open_payload, . - open_payload
+	.size	close_payload, . - close_payload
+
 /****************************************************************************
  * uninstall
  *