s390/boot: Introduce jump_to_kernel() function

Introduce a global function that jumps from the decompressor to the
decompressed kernel. Put its address into svc_old_psw, from where GDB
can take it without loading decompressor symbols. It should be
available throughout the entire decompressor execution, because it's
placed there statically, and nothing in the decompressor uses the SVC
instruction.

Acked-by: Heiko Carstens <[e-mail]>
Signed-off-by: Ilya Leoshkevich <[e-mail]>
Tested-by: Alexander Gordeev <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexander Gordeev <[e-mail]>

arch/s390/boot/Makefile

···
 obj-y	:= head.o als.o startup.o physmem_info.o ipl_parm.o ipl_report.o vmem.o
 obj-y	+= string.o ebcdic.o sclp_early_core.o mem.o ipl_vmparm.o cmdline.o
 obj-y	+= version.o pgm_check.o ctype.o ipl_data.o relocs.o alternative.o
-obj-y	+= uv.o printk.o
+obj-y	+= uv.o printk.o trampoline.o
 obj-$(CONFIG_RANDOMIZE_BASE)	+= kaslr.o
 obj-y	+= $(if $(CONFIG_KERNEL_UNCOMPRESSED),,decompressor.o) info.o
 obj-$(CONFIG_KERNEL_ZSTD) += clz_ctz.o

arch/s390/boot/boot.h

···
 void error(char *m);
 int get_random(unsigned long limit, unsigned long *value);
 void boot_rb_dump(void);
+void __noreturn jump_to_kernel(psw_t *psw);
 
 #ifndef boot_fmt
 #define boot_fmt(fmt)	fmt

arch/s390/boot/ipl_data.c

···
 	struct ccw0	ccwpgm[2];			/* 0x0008 */
 	u8		fill[56];			/* 0x0018 */
 	struct ccw0	ccwpgmcc[20];			/* 0x0050 */
-	u8		pad_0xf0[0x01a0-0x00f0];	/* 0x00f0 */
+	u8		pad_0xf0[0x0140-0x00f0];	/* 0x00f0 */
+	psw_t		svc_old_psw;			/* 0x0140 */
+	u8		pad_0x150[0x01a0-0x0150];	/* 0x0150 */
 	psw_t		restart_psw;			/* 0x01a0 */
 	psw_t		external_new_psw;		/* 0x01b0 */
 	psw_t		svc_new_psw;			/* 0x01c0 */
···
 		[18] = CCW0(CCW_CMD_READ_IPL, 0x690, 0x50, CCW_FLAG_SLI | CCW_FLAG_CC),
 		[19] = CCW0(CCW_CMD_READ_IPL, 0x6e0, 0x50, CCW_FLAG_SLI),
 	},
+	/*
+	 * Let the GDB's lx-symbols command find the jump_to_kernel symbol
+	 * without having to load decompressor symbols.
+	 */
+	.svc_old_psw	  = { .mask = 0, .addr = (unsigned long)jump_to_kernel },
 	.restart_psw	  = { .mask = 0, .addr = IPL_START, },
 	.external_new_psw = { .mask = PSW_MASK_DISABLED, .addr = __LC_EXT_NEW_PSW, },
 	.svc_new_psw	  = { .mask = PSW_MASK_DISABLED, .addr = __LC_SVC_NEW_PSW, },

arch/s390/boot/startup.c

···
 	psw.addr = __kaslr_offset + vmlinux.entry;
 	psw.mask = PSW_KERNEL_BITS;
 	boot_debug("Starting kernel at:  0x%016lx\n", psw.addr);
-	__load_psw(psw);
+	jump_to_kernel(&psw);
 }

arch/s390/boot/trampoline.S

···
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/linkage.h>
+
+# This function is identical to __load_psw(), but the lx-symbols GDB command
+# puts a breakpoint on it, so it needs to be kept separate.
+SYM_CODE_START(jump_to_kernel)
+	lpswe 0(%r2)
+SYM_CODE_END(jump_to_kernel)