[pxeprefix] Work around bug in Etherboot 5.4 when loading undionly.kpxe

Etherboot 5.4 erroneously treats PXENV_UNLOAD_STACK as the "final
shutdown" call, and unhooks INT15.  When using gPXE's undionly.kpxe,
this results in gPXE overwriting the portion of Etherboot located in
high memory, because it is no longer hidden from the system memory map
at the time that gPXE loads.

Work around this by explicitly testing for Etherboot as the underlying
PXE stack (as is already done in undinet.c) and skipping the call to
PXENV_UNLOAD_STACK if necessary.

src/arch/i386/prefix/pxeprefix.S

 #define PXENV_UNDI_SHUTDOWN		0x0005
 #define	PXENV_UNDI_GET_NIC_TYPE		0x0012
+#define PXENV_UNDI_GET_IFACE_INFO	0x0013
 #define	PXENV_STOP_UNDI			0x0015
 #define PXENV_UNLOAD_STACK		0x0070
 
+#define PXE_HACK_EB54			0x0001
+
 	.text
 	.arch i386
 	.org 0
 #include <undi.h>
 
 #define STACK_MAGIC ( 'L' + ( 'R' << 8 ) + ( 'E' << 16 ) + ( 'T' << 24 ) )
+#define EB_MAGIC_1 ( 'E' + ( 't' << 8 ) + ( 'h' << 16 ) + ( 'e' << 24 ) )
+#define EB_MAGIC_2 ( 'r' + ( 'b' << 8 ) + ( 'o' << 16 ) + ( 'o' << 24 ) )
 
 /*****************************************************************************
  * Entry point:	set operating context, print welcome message
 	movw	$10f, %si
 	call	print_message
 	call	print_pci_busdevfn
-	movb	$0x0a, %al
-	call	print_character
 	jmp	99f
 	.section ".prefix.data", "aw", @progbits
 10:	.asciz	"         UNDI device is PCI "
 	movw	$10f, %si
 	call	print_message
 	.section ".prefix.data", "aw", @progbits
-10:	.asciz	"         Unable to determine UNDI physical device\n"
+10:	.asciz	"         Unable to determine UNDI physical device"
 	.previous
 
 99:
 
+/*****************************************************************************
+ * Determine interface type
+ *****************************************************************************
+ */
+get_iface_type:
+	/* Issue PXENV_UNDI_GET_IFACE_INFO */
+	movw	$PXENV_UNDI_GET_IFACE_INFO, %bx
+	call	pxe_call
+	jnc	1f
+	call	print_pxe_error
+	jmp	99f
+1:	/* Print interface type */
+	movw	$10f, %si
+	call	print_message
+	leaw	( pxe_parameter_structure + 0x02 ), %si
+	call	print_message
+	.section ".prefix.data", "aw", @progbits
+10:	.asciz	", type "
+	.previous
+	/* Check for "Etherboot" interface type */
+	cmpl	$EB_MAGIC_1, ( pxe_parameter_structure + 0x02 )
+	jne	99f
+	cmpl	$EB_MAGIC_2, ( pxe_parameter_structure + 0x06 )
+	jne	99f
+	movw	$10f, %si
+	call	print_message
+	.section ".prefix.data", "aw", @progbits
+10:	.asciz	" (workaround enabled)"
+	.previous
+	/* Flag Etherboot workarounds as required */
+	orw	$PXE_HACK_EB54, pxe_hacks
+
+99:	movb	$0x0a, %al
+	call	print_character
+
 /*****************************************************************************
  * Leave NIC in a safe state
  *****************************************************************************
 	call	print_pxe_error
 1:
 unload_base_code:
+	/* Etherboot treats PXENV_UNLOAD_STACK as PXENV_STOP_UNDI, so
+	 * we must not issue this call if the underlying stack is
+	 * Etherboot and we were not intending to issue a PXENV_STOP_UNDI.
+	 */
+#ifdef PXELOADER_KEEP_UNDI
+	testw	$PXE_HACK_EB54, pxe_hacks
+	jnz	99f
+#endif /* PXELOADER_KEEP_UNDI */
 	/* Issue PXENV_UNLOAD_STACK */
 	movw	$PXENV_UNLOAD_STACK, %bx
 	call	pxe_call
 	testw	%ax, %ax
 	jz	1f
 	stc
-1:	/* Restore registers and return */
+1:	/* Clear direction flag, for the sake of sanity */
+	cld
+	/* Restore registers and return */
 	popw	%es
 	popw	%di
 	ret
 pxe_esp:		.long 0
 pxe_ss:			.word 0
 
-pxe_parameter_structure: .fill 20
+pxe_parameter_structure: .fill 64
 
 undi_code_segoff:
 undi_code_size:		.word 0
 undi_data_size:		.word 0
 undi_data_segment:	.word 0
 
+pxe_hacks:		.word 0
+
 /* The following fields are part of a struct undi_device */
 
 undi_device: