On s390 64-bit the s390x ELF ABI [1] designates register 11 as the
"preferred" frame pointer (FP) register in user space.

While at it convert instruction_pointer() and user_stack_pointer()
from macros to inline functions, to align their definition with
x86 and arm64.

Use const qualifier on struct pt_regs pointers to prevent compiler
warnings:

arch/s390/kernel/stacktrace.c: In function ‘arch_stack_walk_user_common’:
arch/s390/kernel/stacktrace.c:114:34: warning: passing argument 1 of
‘instruction_pointer’ discards ‘const’ qualifier from pointer target
type [-Wdiscarded-qualifiers]
...
arch/s390/kernel/stacktrace.c:117:48: warning: passing argument 1 of
‘user_stack_pointer’ discards ‘const’ qualifier from pointer target
type [-Wdiscarded-qualifiers]
...

[1]: s390x ELF ABI, https://github.com/IBM/s390x-abi/releases

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
    Changes in RFC v2:
    - Separate provide frame_pointer() into this new commit.

 arch/s390/include/asm/ptrace.h | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/s390/include/asm/ptrace.h b/arch/s390/include/asm/ptrace.h
index dfa770b15fad..455c119167fc 100644
--- a/arch/s390/include/asm/ptrace.h
+++ b/arch/s390/include/asm/ptrace.h
@@ -212,8 +212,6 @@ void update_cr_regs(struct task_struct *task);
 #define arch_has_block_step()	(1)
 
 #define user_mode(regs) (((regs)->psw.mask & PSW_MASK_PSTATE) != 0)
-#define instruction_pointer(regs) ((regs)->psw.addr)
-#define user_stack_pointer(regs)((regs)->gprs[15])
 #define profile_pc(regs) instruction_pointer(regs)
 
 static inline long regs_return_value(struct pt_regs *regs)
@@ -235,6 +233,22 @@ static __always_inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
 	return regs->gprs[15];
 }
 
+static __always_inline unsigned long instruction_pointer(const struct pt_regs *regs)
+{
+	return regs->psw.addr;
+}
+
+static __always_inline unsigned long frame_pointer(const struct pt_regs *regs)
+{
+	/* Return ABI-designated "preferred" frame-pointer register value. */
+	return regs->gprs[11];
+}
+
+static __always_inline unsigned long user_stack_pointer(const struct pt_regs *regs)
+{
+	return regs->gprs[15];
+}
+
 static __always_inline unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset)
 {
 	if (offset >= NUM_GPRS)
-- 
2.51.0