[romprefix] On a PCI3.0, non-BBS system, use the correct %cs for INT19

On a system that doesn't support BBS, we end up hooking INT19 to gain
control of the boot process.  If the system is PCI3.0, we must take
care to use the runtime value for %cs, rather than the POST-time
value, otherwise we end up pointing INT19 to the temporary option ROM
POST scratch area.

src/arch/i386/prefix/romprefix.S

 	popw	%ds
 	pushw	$0x40
 	popw	%fs
+
 	/* 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:
 	 */
 	movw	%bx, %gs
 	movw	%di, %bx
+
 	/* Print message as early as possible */
 	movw	$init_message, %si
 	xorw	%di, %di
 	call	print_message
 	call	print_pci_busdevfn
+
 	/* Fill in product name string, if possible */
 	movw	$prodstr_pci_id, %di
 	call	print_pci_busdevfn
 	movb	$' ', prodstr_separator
+
 	/* Print segment address */
 	movb	$' ', %al
 	xorw	%di, %di
 	call	print_character
 	movw	%cs, %ax
 	call	print_hex_word
+
+	/* Check for PCI BIOS version */
+	pushl	%ebx
+	pushl	%edx
+	stc
+	movw	$0xb101, %ax
+	int	$0x1a
+	jc	1f
+	cmpl	$PCI_SIGNATURE, %edx
+	jne	1f
+	testb	%ah, %ah
+	jnz	1f
+	movw	$init_message_pci, %si
+	xorw	%di, %di
+	call	print_message
+	movb	%bh, %al
+	call	print_hex_nibble
+	movb	$'.', %al
+	call	print_character
+	movb	%bl, %al
+	call	print_hex_byte
+	cmpb	$3, %bh
+	jae	2f
+1:	/* PCI <3.0: set %gs (runtime segment) = %cs (init-time segment) */
+	pushw	%cs
+	popw	%gs
+2:	popl	%edx
+	popl	%ebx
+
 	/* Check for PnP BIOS */
 	testw	$0x0f, %bx	/* PnP signature must be aligned - bochs    */
 	jnz	hook_int19	/* uses unalignment to indicate 'fake' PnP. */
 	movw	$init_message_pnp, %si
 	xorw	%di, %di
 	call	print_message
+
 	/* Check for BBS */
 	pushw	%es:0x1b(%bx)	/* Real-mode data segment */
 	pushw	%ds		/* &(bbs_version) */
 	movw	%ax, %es
 	pushl	%es:( 0x19 * 4 )
 	popl	orig_int19
-	pushw	%cs
+	pushw	%gs /* %gs contains runtime %cs */
 	pushw	$int19_entry
 	popl	%es:( 0x19 * 4 )
 hook_bbs:
+
 	/* Check for PMM */
 	movw	$( 0xe000 - 1 ), %bx
 pmm_scan:
 	subb	%bl, checksum
 	popal
 no_pmm:
-	/* Check for PCI BIOS */
-	pushl	%edx
-	stc
-	movw	$0xb101, %ax
-	int	$0x1a
-	jc	no_pci
-	cmpl	$PCI_SIGNATURE, %edx
-	popl	%edx
-	jne	no_pci
-	testb	%ah, %ah
-	jnz	no_pci
-	movw	$init_message_pci, %si
-	xorw	%di, %di
-	call	print_message
-	movb	%bh, %al
-	call	print_hex_nibble
-	movb	$'.', %al
-	call	print_character
-	movb	%bl, %al
-	call	print_hex_byte
-	cmpb	$3, %bh
-	jb	no_pci3
-	/* Copy self to option ROM space (required for PCI3.0) */
+
+	/* Copy self to option ROM space.  Required for PCI3.0, which
+	 * loads us to a temporary location in low memory.  Will be a
+	 * no-op for lower PCI versions.
+	 */
 	movb	$' ', %al
 	xorw	%di, %di
 	call	print_character
 	xorw	%si, %si
 	xorw	%di, %di
 	cs rep	movsb
-no_pci3:
-no_pci:
+
 	/* Prompt for POST-time shell */
 	movw	$init_message_prompt, %si
 	xorw	%di, %di
 	pushw	%cs
 	call	exec
 no_key_pressed:
+
 	/* Print blank lines to terminate messages */
 	movw	$init_message_end, %si
 	xorw	%di, %di
 	call	print_message
+
 	/* Restore registers */
 	popw	%gs
 	popw	%fs
 	popw	%es
 	popw	%ds
 	popaw
+
 	/* Indicate boot capability to PnP BIOS, if present */
 	movw	$0x20, %ax
 	lret