s390/head: fix error message on unsupported hardware

startup calls the C function _sclp_print_early() if the machine we're
running on is not supported by the kernel. sclp.c is getting built
with -m64, so _sclp_print_early() expects the zSeries ELF ABI to be
used.

We previously called _sclp_print_early() using the S/390 ELF ABI, with
a stack frame size of 96 bytes and while being in 31-bit address
mode. This caused _sclp_wait_int() (called indirectly from
_sclp_print_early()) to jump to an undefined address. While
_sclp_wait_int() contained some code to deal with being called in
31-bit addressing mode, it didn't quite work. While fixing this is
possible, the code would still only work by chance and could break any
time.

Ensure compliance with the zSeries ELF ABI by switching to 64-bit
addressing mode early and using a minimum stack frame size of 160
bytes.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/head.S

@@ -26,6 +26,7 @@
 #include <asm/asm-offsets.h>
 #include <asm/thread_info.h>
 #include <asm/page.h>
+#include <asm/ptrace.h>
 
 #define ARCH_OFFSET	4
 
@@ -364,7 +365,7 @@ ENTRY(startup_kdump)
 	bras	%r13,0f
 	.fill	16,4,0x0
 0:	lmh	%r0,%r15,0(%r13)	# clear high-order half of gprs
-	sam31				# switch to 31 bit addressing mode
+	sam64				# switch to 64 bit addressing mode
 	basr	%r13,0			# get base
 .LPG0:
 	xc	0x200(256),0x200	# partially clear lowcore
@@ -395,7 +396,7 @@ ENTRY(startup_kdump)
 	jnz	1b
 	j	4f
 2:	l	%r15,.Lstack-.LPG0(%r13)
-	ahi	%r15,-96
+	ahi	%r15,-STACK_FRAME_OVERHEAD
 	la	%r2,.Lals_string-.LPG0(%r13)
 	l	%r3,.Lsclp_print-.LPG0(%r13)
 	basr	%r14,%r3
@@ -429,8 +430,7 @@ ENTRY(startup_kdump)
 	.long 1, 0xc0000000
 #endif
 4:
-	/* Continue with 64bit startup code in head64.S */
-	sam64				# switch to 64 bit mode
+	/* Continue with startup code in head64.S */
 	jg	startup_continue
 
 	.align	8