s390/ftrace: assume -mhotpatch or -mrecord-mcount always available

Currently the kernel minimal compiler requirement is gcc 4.9 or
clang 10.0.1.
* gcc -mhotpatch option is supported since 4.8.
* A combination of -pg -mrecord-mcount -mnop-mcount -mfentry flags is
supported since gcc 9 and since clang 10.

Drop support for old -pg function prologues. Which leaves binary
compatible -mhotpatch / -mnop-mcount -mfentry prologues in a form:
	brcl	0,0
Which are also do not require initial nop optimization / conversion and
presence of _mcount symbol.

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

3 files changed, 24 insertions, 78 deletions

diff --git a/arch/s390/include/asm/ftrace.h b/arch/s390/include/asm/ftrace.h
index 68d362f8d6c1..695c61989f97 100644
--- a/arch/s390/include/asm/ftrace.h
+++ b/arch/s390/include/asm/ftrace.h
@@ -2,16 +2,9 @@
 #ifndef _ASM_S390_FTRACE_H
 #define _ASM_S390_FTRACE_H
 
+#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 #define ARCH_SUPPORTS_FTRACE_OPS 1
-
-#if defined(CC_USING_HOTPATCH) || defined(CC_USING_NOP_MCOUNT)
 #define MCOUNT_INSN_SIZE	6
-#else
-#define MCOUNT_INSN_SIZE	24
-#define MCOUNT_RETURN_FIXUP	18
-#endif
-
-#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 
 #ifndef __ASSEMBLY__
 
@@ -22,7 +15,6 @@
 #define ftrace_return_address(n) __builtin_return_address(n)
 #endif
 
-void _mcount(void);
 void ftrace_caller(void);
 
 extern char ftrace_graph_caller_end;
@@ -30,12 +22,20 @@ extern unsigned long ftrace_plt;
 
 struct dyn_arch_ftrace { };
 
-#define MCOUNT_ADDR ((unsigned long)_mcount)
+#define MCOUNT_ADDR 0
 #define FTRACE_ADDR ((unsigned long)ftrace_caller)
 
 #define KPROBE_ON_FTRACE_NOP	0
 #define KPROBE_ON_FTRACE_CALL	1
 
+struct module;
+struct dyn_ftrace;
+/*
+ * Either -mhotpatch or -mnop-mcount is used - no explicit init is required
+ */
+static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec) { return 0; }
+#define ftrace_init_nop ftrace_init_nop
+
 static inline unsigned long ftrace_call_adjust(unsigned long addr)
 {
 	return addr;
@@ -49,28 +49,17 @@ struct ftrace_insn {
 static inline void ftrace_generate_nop_insn(struct ftrace_insn *insn)
 {
 #ifdef CONFIG_FUNCTION_TRACER
-#if defined(CC_USING_HOTPATCH) || defined(CC_USING_NOP_MCOUNT)
 	/* brcl 0,0 */
 	insn->opc = 0xc004;
 	insn->disp = 0;
-#else
-	/* jg .+24 */
-	insn->opc = 0xc0f4;
-	insn->disp = MCOUNT_INSN_SIZE / 2;
-#endif
 #endif
 }
 
 static inline int is_ftrace_nop(struct ftrace_insn *insn)
 {
 #ifdef CONFIG_FUNCTION_TRACER
-#if defined(CC_USING_HOTPATCH) || defined(CC_USING_NOP_MCOUNT)
 	if (insn->disp == 0)
 		return 1;
-#else
-	if (insn->disp == MCOUNT_INSN_SIZE / 2)
-		return 1;
-#endif
 #endif
 	return 0;
 }
diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
index b388e87a08bf..ebc1284a618b 100644
--- a/arch/s390/kernel/ftrace.c
+++ b/arch/s390/kernel/ftrace.c
@@ -22,56 +22,26 @@
 #include "entry.h"
 
 /*
- * The mcount code looks like this:
- *	stg	%r14,8(%r15)		# offset 0
- *	larl	%r1,<&counter>		# offset 6
- *	brasl	%r14,_mcount		# offset 12
- *	lg	%r14,8(%r15)		# offset 18
- * Total length is 24 bytes. Only the first instruction will be patched
- * by ftrace_make_call / ftrace_make_nop.
- * The enabled ftrace code block looks like this:
+ * To generate function prologue either gcc's hotpatch feature (since gcc 4.8)
+ * or a combination of -pg -mrecord-mcount -mnop-mcount -mfentry flags
+ * (since gcc 9 / clang 10) is used.
+ * In both cases the original and also the disabled function prologue contains
+ * only a single six byte instruction and looks like this:
+ * >	brcl	0,0			# offset 0
+ * To enable ftrace the code gets patched like above and afterwards looks
+ * like this:
  * >	brasl	%r0,ftrace_caller	# offset 0
- *	larl	%r1,<&counter>		# offset 6
- *	brasl	%r14,_mcount		# offset 12
- *	lg	%r14,8(%r15)		# offset 18
+ *
+ * The instruction will be patched by ftrace_make_call / ftrace_make_nop.
  * The ftrace function gets called with a non-standard C function call ABI
  * where r0 contains the return address. It is also expected that the called
  * function only clobbers r0 and r1, but restores r2-r15.
  * For module code we can't directly jump to ftrace caller, but need a
  * trampoline (ftrace_plt), which clobbers also r1.
- * The return point of the ftrace function has offset 24, so execution
- * continues behind the mcount block.
- * The disabled ftrace code block looks like this:
- * >	jg	.+24			# offset 0
- *	larl	%r1,<&counter>		# offset 6
- *	brasl	%r14,_mcount		# offset 12
- *	lg	%r14,8(%r15)		# offset 18
- * The jg instruction branches to offset 24 to skip as many instructions
- * as possible.
- * In case we use gcc's hotpatch feature the original and also the disabled
- * function prologue contains only a single six byte instruction and looks
- * like this:
- * >	brcl	0,0			# offset 0
- * To enable ftrace the code gets patched like above and afterwards looks
- * like this:
- * >	brasl	%r0,ftrace_caller	# offset 0
  */
 
 unsigned long ftrace_plt;
 
-static inline void ftrace_generate_orig_insn(struct ftrace_insn *insn)
-{
-#if defined(CC_USING_HOTPATCH) || defined(CC_USING_NOP_MCOUNT)
-	/* brcl 0,0 */
-	insn->opc = 0xc004;
-	insn->disp = 0;
-#else
-	/* stg r14,8(r15) */
-	insn->opc = 0xe3e0;
-	insn->disp = 0xf0080024;
-#endif
-}
-
 int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
 		       unsigned long addr)
 {
@@ -85,15 +55,10 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 
 	if (copy_from_kernel_nofault(&old, (void *) rec->ip, sizeof(old)))
 		return -EFAULT;
-	if (addr == MCOUNT_ADDR) {
-		/* Initial code replacement */
-		ftrace_generate_orig_insn(&orig);
-		ftrace_generate_nop_insn(&new);
-	} else {
-		/* Replace ftrace call with a nop. */
-		ftrace_generate_call_insn(&orig, rec->ip);
-		ftrace_generate_nop_insn(&new);
-	}
+	/* Replace ftrace call with a nop. */
+	ftrace_generate_call_insn(&orig, rec->ip);
+	ftrace_generate_nop_insn(&new);
+
 	/* Verify that the to be replaced code matches what we expect. */
 	if (memcmp(&orig, &old, sizeof(old)))
 		return -EINVAL;
diff --git a/arch/s390/kernel/mcount.S b/arch/s390/kernel/mcount.S
index 7458dcfd6464..faf64c2f90f5 100644
--- a/arch/s390/kernel/mcount.S
+++ b/arch/s390/kernel/mcount.S
@@ -33,11 +33,6 @@ ENDPROC(ftrace_stub)
 #define TRACED_FUNC_FRAME_SIZE	STACK_FRAME_OVERHEAD
 #endif
 
-ENTRY(_mcount)
-	BR_EX	%r14
-ENDPROC(_mcount)
-EXPORT_SYMBOL(_mcount)
-
 ENTRY(ftrace_caller)
 	.globl	ftrace_regs_caller
 	.set	ftrace_regs_caller,ftrace_caller
@@ -46,9 +41,6 @@ ENTRY(ftrace_caller)
 	ipm	%r14				# don't put any instructions
 	sllg	%r14,%r14,16			# clobbering CC before this point
 	lgr	%r1,%r15
-#if !(defined(CC_USING_HOTPATCH) || defined(CC_USING_NOP_MCOUNT))
-	aghi	%r0,MCOUNT_RETURN_FIXUP
-#endif
 	# allocate stack frame for ftrace_caller to contain traced function
 	aghi	%r15,-TRACED_FUNC_FRAME_SIZE
 	stg	%r1,__SF_BACKCHAIN(%r15)