s390: always use the packed stack layout

-mpacked-stack option has been supported by both minimum
gcc and clang versions for a while. With commit e2bc3e91d9
("scripts/min-tool-version.sh: Raise minimum clang version to 13.0.0
for s390") minimum clang version now also supports a combination
of flags -mpacked-stack -mbackchain -pg -mfentry and fulfills
all requirements to always enable the packed stack layout.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/Kconfig

@@ -155,7 +155,7 @@ config S390
 	select HAVE_DYNAMIC_FTRACE_WITH_ARGS
 	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS
-	select HAVE_EBPF_JIT if PACK_STACK && HAVE_MARCH_Z196_FEATURES
+	select HAVE_EBPF_JIT if HAVE_MARCH_Z196_FEATURES
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS
 	select HAVE_FAST_GUP
 	select HAVE_FENTRY
@@ -656,20 +656,6 @@ config MAX_PHYSMEM_BITS
 	  Increasing the number of bits also increases the kernel image size.
 	  By default 46 bits (64TB) are supported.
 
-config PACK_STACK
-	def_bool y
-	prompt "Pack kernel stack"
-	help
-	  This option enables the compiler option -mkernel-backchain if it
-	  is available. If the option is available the compiler supports
-	  the new stack layout which dramatically reduces the minimum stack
-	  frame size. With an old compiler a non-leaf function needs a
-	  minimum of 96 bytes on 31 bit and 160 bytes on 64 bit. With
-	  -mkernel-backchain the minimum size drops to 16 byte on 31 bit
-	  and 24 byte on 64 bit.
-
-	  Say Y if you are unsure.
-
 config CHECK_STACK
 	def_bool y
 	depends on !VMAP_STACK

arch/s390/Makefile

@@ -21,7 +21,7 @@ endif
 aflags_dwarf	:= -Wa,-gdwarf-2
 KBUILD_AFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -D__ASSEMBLY__
 KBUILD_AFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO),$(aflags_dwarf))
-KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2
+KBUILD_CFLAGS_DECOMPRESSOR := $(CLANG_FLAGS) -m64 -O2 -mpacked-stack
 KBUILD_CFLAGS_DECOMPRESSOR += -DDISABLE_BRANCH_PROFILING -D__NO_FORTIFY
 KBUILD_CFLAGS_DECOMPRESSOR += -fno-delete-null-pointer-checks -msoft-float -mbackchain
 KBUILD_CFLAGS_DECOMPRESSOR += -fno-asynchronous-unwind-tables
@@ -68,11 +68,6 @@ cflags-y += -Wa,-I$(srctree)/arch/$(ARCH)/include
 #
 cflags-$(CONFIG_FRAME_POINTER) += -fno-optimize-sibling-calls
 
-ifneq ($(call cc-option,-mpacked-stack -mbackchain -msoft-float),)
-cflags-$(CONFIG_PACK_STACK)  += -mpacked-stack -D__PACK_STACK
-aflags-$(CONFIG_PACK_STACK)  += -D__PACK_STACK
-endif
-
 KBUILD_AFLAGS_DECOMPRESSOR += $(aflags-y)
 KBUILD_CFLAGS_DECOMPRESSOR += $(cflags-y)
 
@@ -111,7 +106,7 @@ endif
 # Test CFI features of binutils
 cfi := $(call as-instr,.cfi_startproc\n.cfi_val_offset 15$(comma)-160\n.cfi_endproc,-DCONFIG_AS_CFI_VAL_OFFSET=1)
 
-KBUILD_CFLAGS	+= -mbackchain -msoft-float $(cflags-y)
+KBUILD_CFLAGS	+= -mpacked-stack -mbackchain -msoft-float $(cflags-y)
 KBUILD_CFLAGS	+= -pipe -Wno-sign-compare
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables $(cfi)
 KBUILD_AFLAGS	+= $(aflags-y) $(cfi)

arch/s390/include/asm/stacktrace.h

@@ -36,22 +36,14 @@ static inline bool on_stack(struct stack_info *info,
 
 /*
  * Stack layout of a C stack frame.
+ * Kernel uses the packed stack layout (-mpacked-stack).
  */
-#ifndef __PACK_STACK
-struct stack_frame {
-	unsigned long back_chain;
-	unsigned long empty1[5];
-	unsigned long gprs[10];
-	unsigned int  empty2[8];
-};
-#else
 struct stack_frame {
 	unsigned long empty1[5];
 	unsigned int  empty2[8];
 	unsigned long gprs[10];
 	unsigned long back_chain;
 };
-#endif
 
 /*
  * Unlike current_stack_pointer() which simply returns current value of %r15

arch/s390/kernel/mcount.S

@@ -26,12 +26,8 @@ ENDPROC(ftrace_stub)
 #define STACK_PTREGS_PSW	(STACK_PTREGS + __PT_PSW)
 #define STACK_PTREGS_ORIG_GPR2	(STACK_PTREGS + __PT_ORIG_GPR2)
 #define STACK_PTREGS_FLAGS	(STACK_PTREGS + __PT_FLAGS)
-#ifdef __PACK_STACK
+/* packed stack: allocate just enough for r14, r15 and backchain */
-/* allocate just enough for r14, r15 and backchain */
 #define TRACED_FUNC_FRAME_SIZE	24
-#else
-#define TRACED_FUNC_FRAME_SIZE	STACK_FRAME_OVERHEAD
-#endif
 
 	.macro	ftrace_regs_entry, allregs=0
 	stg	%r14,(__SF_GPRS+8*8)(%r15)	# save traced function caller

arch/s390/net/bpf_jit_comp.c

@@ -7,7 +7,6 @@
  *  - HAVE_MARCH_Z196_FEATURES: laal, laalg
  *  - HAVE_MARCH_Z10_FEATURES: msfi, cgrj, clgrj
  *  - HAVE_MARCH_Z9_109_FEATURES: alfi, llilf, clfi, oilf, nilf
- *  - PACK_STACK
  *  - 64BIT
  *
  * Copyright IBM Corp. 2012,2015