s390: remove decompressor's head.S

Decompressor's head.S provided "data mover" sole purpose of which has
been to safely move uncompressed kernel at 0x100000 and jump to it.

With current bzImage layout entire decompressor's code guaranteed to be
in a safe location under 0x100000, and hence could not be overwritten
during kernel move. For that reason head.S could be replaced with simple
memmove function. To do so introduce early boot code phase which is
executed from arch/s390/boot/head.S after "verify_facilities" and takes
care of optional kernel image decompression and transition to it.

Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/Makefile

@@ -27,7 +27,7 @@ endif
 
 CFLAGS_sclp_early_core.o += -I$(srctree)/drivers/s390/char
 
-obj-y	:= head.o als.o ebcdic.o sclp_early_core.o mem.o
+obj-y	:= head.o als.o startup.o ebcdic.o sclp_early_core.o mem.o
 targets	:= bzImage startup.a $(obj-y)
 subdir-	:= compressed
 

arch/s390/boot/boot.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef BOOT_BOOT_H
+#define BOOT_BOOT_H
+
+void startup_kernel(void);
+
+#endif /* BOOT_BOOT_H */

arch/s390/boot/compressed/Makefile

@@ -9,7 +9,7 @@ KCOV_INSTRUMENT := n
 GCOV_PROFILE := n
 UBSAN_SANITIZE := n
 
-obj-y	:= $(if $(CONFIG_KERNEL_UNCOMPRESSED),,head.o misc.o) piggy.o
+obj-y	:= $(if $(CONFIG_KERNEL_UNCOMPRESSED),,misc.o) piggy.o
 targets	:= vmlinux.lds vmlinux vmlinux.bin vmlinux.bin.gz vmlinux.bin.bz2
 targets += vmlinux.bin.xz vmlinux.bin.lzma vmlinux.bin.lzo vmlinux.bin.lz4
 targets += vmlinux.scr.lds $(obj-y) $(if $(CONFIG_KERNEL_UNCOMPRESSED),,sizes.h)
@@ -32,9 +32,6 @@ quiet_cmd_sizes = GEN     $@
 $(obj)/sizes.h: vmlinux
 	$(call if_changed,sizes)
 
-AFLAGS_head.o += -I$(objtree)/$(obj)
-$(obj)/head.o: $(obj)/sizes.h
-
 CFLAGS_misc.o += -I$(objtree)/$(obj)
 $(obj)/misc.o: $(obj)/sizes.h
 

arch/s390/boot/compressed/decompressor.h

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef BOOT_COMPRESSED_DECOMPRESSOR_H
+#define BOOT_COMPRESSED_DECOMPRESSOR_H
+
+#ifdef CONFIG_KERNEL_UNCOMPRESSED
+static inline void *decompress_kernel(unsigned long *uncompressed_size) {}
+#else
+void *decompress_kernel(unsigned long *uncompressed_size);
+#endif
+
+#endif /* BOOT_COMPRESSED_DECOMPRESSOR_H */

arch/s390/boot/compressed/misc.c

@@ -12,6 +12,7 @@
 #include <asm/sclp.h>
 #include <asm/ipl.h>
 #include "sizes.h"
+#include "decompressor.h"
 
 /*
  * gzip declarations
@@ -82,7 +83,7 @@ static void error(char *x)
 	asm volatile("lpsw %0" : : "Q" (psw));
 }
 
-unsigned long decompress_kernel(void)
+void *decompress_kernel(unsigned long *uncompressed_size)
 {
 	void *output, *kernel_end;
 
@@ -111,6 +112,8 @@ unsigned long decompress_kernel(void)
 	free_mem_end_ptr = free_mem_ptr + HEAP_SIZE;
 
 	__decompress(input_data, input_len, NULL, NULL, output, 0, NULL, error);
-	return (unsigned long) output;
+	if (uncompressed_size)
+		*uncompressed_size = SZ__bss_start;
+	return output;
 }
 

arch/s390/boot/head.S

@@ -312,11 +312,7 @@ ENTRY(startup_kdump)
 	mvc	__LC_LAST_UPDATE_TIMER(8),6f-.LPG0(%r13)
 	l	%r15,.Lstack-.LPG0(%r13)
 	brasl	%r14,verify_facilities
-#ifdef CONFIG_KERNEL_UNCOMPRESSED
-	jg	startup_continue
-#else
-	jg	startup_decompressor
-#endif
+	brasl	%r14,startup_kernel
 
 .Lstack:
 	.long	0x8000 + THREAD_SIZE - STACK_FRAME_OVERHEAD

arch/s390/boot/startup.c

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/string.h>
+#include "compressed/decompressor.h"
+#include "boot.h"
+
+void startup_kernel(void)
+{
+	void (*startup_continue)(void) = (void *)0x100000;
+	unsigned long uncompressed_size;
+	void *uncompressed_img;
+
+	if (!IS_ENABLED(CONFIG_KERNEL_UNCOMPRESSED)) {
+		uncompressed_img = decompress_kernel(&uncompressed_size);
+		memmove(startup_continue, uncompressed_img, uncompressed_size);
+	}
+	startup_continue();
+}