s390/decompressor: trim uncompressed image head during the build

It seems to be suboptimal to compress the entire image, so that we
could decompress and throw away the first 68k (0x11000) during the boot
process. This patch trims 0x11000 bytes during the build process and
adjusts symbols in size.h accordingly.

Besides making bzImage smaller, this change would also be a benefit
for kaslr. It allows to decompress the kernel right to desired offset,
without worrying about dead weight in front of the decompressed image.
Signed-off-by: Vasily Gorbik <gor@linux.vnet.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/compressed/Makefile

@@ -29,11 +29,16 @@ LDFLAGS_vmlinux := --oformat $(LD_BFD) -e startup -T
 $(obj)/vmlinux: $(obj)/vmlinux.lds $(OBJECTS)
 	$(call if_changed,ld)
 
-sed-sizes := -e 's/^\([0-9a-fA-F]*\) . \(__bss_start\|_end\)$$/\#define SZ\2 0x\1/p'
+TRIM_HEAD_SIZE := 0x11000
+
+sed-sizes := -e 's/^\([0-9a-fA-F]*\) . \(__bss_start\|_end\)$$/\#define SZ\2 (0x\1 - $(TRIM_HEAD_SIZE))/p'
 
 quiet_cmd_sizes = GEN     $@
       cmd_sizes = $(NM) $< | sed -n $(sed-sizes) > $@
 
+quiet_cmd_trim_head = TRIM    $@
+      cmd_trim_head = tail -c +$$(($(TRIM_HEAD_SIZE) + 1)) $< > $@
+
 $(obj)/sizes.h: vmlinux
 	$(call if_changed,sizes)
 
@@ -43,10 +48,13 @@ $(obj)/head.o: $(obj)/sizes.h
 CFLAGS_misc.o += -I$(objtree)/$(obj)
 $(obj)/misc.o: $(obj)/sizes.h
 
-OBJCOPYFLAGS_vmlinux.bin :=  -R .comment -S
-$(obj)/vmlinux.bin: vmlinux
+OBJCOPYFLAGS_vmlinux.bin.full :=  -R .comment -S
+$(obj)/vmlinux.bin.full: vmlinux
 	$(call if_changed,objcopy)
 
+$(obj)/vmlinux.bin: $(obj)/vmlinux.bin.full
+	$(call if_changed,trim_head)
+
 vmlinux.bin.all-y := $(obj)/vmlinux.bin
 
 suffix-$(CONFIG_KERNEL_GZIP)  := gz

arch/s390/boot/compressed/head.S

@@ -23,12 +23,10 @@ ENTRY(startup_continue)
 	aghi	%r15,-160
 	brasl	%r14,decompress_kernel
 	# Set up registers for memory mover. We move the decompressed image to
-	# 0x11000, starting at offset 0x11000 in the decompressed image so
-	# that code living at 0x11000 in the image will end up at 0x11000 in
-	# memory.
+	# 0x11000, where startup_continue of the decompressed image is supposed
+	# to be.
 	lgr	%r4,%r2
 	lg	%r2,.Loffset-.LPG1(%r13)
-	la	%r4,0(%r2,%r4)
 	lg	%r3,.Lmvsize-.LPG1(%r13)
 	lgr	%r5,%r3
 	# Move the memory mover someplace safe so it doesn't overwrite itself.