[romprefix] Attempt to gracefully handle semi-PnP IBM BIOSes

Some IBM BIOSes provide partial support for PnP: they will use the BEV
entry point but will not advertise PnP support.  This causes iPXE to
hook INT 19, which disrupts the boot process.

Attempt to improve this situation by detecting an IBM BIOS and
treating it as a PnP BIOS despite the absence of a PnP signature.

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

src/arch/i386/prefix/romprefix.S

 #define PNP_SIGNATURE ( '$' + ( 'P' << 8 ) + ( 'n' << 16 ) + ( 'P' << 24 ) )
 #define PMM_SIGNATURE ( '$' + ( 'P' << 8 ) + ( 'M' << 16 ) + ( 'M' << 24 ) )
 #define PCI_SIGNATURE ( 'P' + ( 'C' << 8 ) + ( 'I' << 16 ) + ( ' ' << 24 ) )
+#define IBM_SIGNATURE ( ( 'I' << 24 ) + ( 'B' << 16 ) + ( 'M' << 8 ) + ' ' )
 #define STACK_MAGIC ( 'L' + ( 'R' << 8 ) + ( 'E' << 16 ) + ( 'T' << 24 ) )
 #define PMM_ALLOCATE 0x0000
 #define PMM_FIND 0x0001
 init:
 	/* Preserve registers, clear direction flag, set %ds=%cs */
 	pushaw
+	pushl	%ebx
 	pushw	%ds
 	pushw	%es
 	pushw	%fs
 	pushw	%cs
 	popw	%ds
 
-	/* Shuffle some registers around.  We need %di available for
-	 * the print_xxx functions, and in a register that's
-	 * addressable from %es, so shuffle as follows:
+	/* Shuffle some registers around.  We need to use %di for the
+	 * print_xxx functions, so shuffle as follows:
 	 *
-	 *    %di (pointer to PnP structure) => %bx
 	 *    %bx (runtime segment address, for PCI 3.0) => %gs
+	 *    %edi (IBM signature) => %ebx
 	 */
 	movw	%bx, %gs
-	movw	%di, %bx
+	movl	%edi, %ebx
 
 	/* Store PCI bus:dev.fn address */
 	movw	%ax, init_pci_busdevfn
 	popl	%edx
 	popl	%ebx
 
-	/* Check for PnP BIOS.  Although %es:di should point to the
+	/* Check for IBM BIOS, which uses the PnP entry points but
+	 * doesn't indicate PnP support.
+	 */
+	cmpl	$IBM_SIGNATURE, %ebx
+	jne	no_ibm
+	movw	$init_message_ibm, %si
+	xorw	%di, %di
+	call	print_message
+	jmp	pnp_done
+no_ibm:	/* Check for PnP BIOS.  Although %es:di should point to the
 	 * PnP BIOS signature on entry, some BIOSes fail to do this.
 	 */
 	movw	$( 0xf000 - 1 ), %bx
 	popw	%fs
 	popw	%es
 	popw	%ds
+	popl	%ebx
 	popaw
 
 	/* Indicate boot capability to PnP BIOS, if present */
 init_message_pci:
 	.asciz	" PCI"
 	.size	init_message_pci, . - init_message_pci
+init_message_ibm:
+	.asciz	" IBM"
+	.size	init_message_ibm, . - init_message_ibm
 init_message_pnp:
 	.asciz	" PnP"
 	.size	init_message_pnp, . - init_message_pnp