arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

PATCH In review Abel Vesa 4 participants 7 messages Dec 8, 2016 – Jan 24, 2017

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Arm (32-bit) ftrace

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Abel Vesa

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2016-12-08 21:46 UTC #0 lore raw

PATCH 1/1 arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH_REGS

From: Jean-Jacques Hiblot <[email protected]>

The DYNAMIC_FTRACE_WITH_REGS configuration makes it possible for a ftrace operation to specify if registers need to saved/restored by the ftrace handler. This is needed by kgraft and possibly other ftrace-based tools, and the ARM architecture is currently lacking this feature. It would also be the first step to support the "Kprobes-on-ftrace" optimization on ARM.

This patch introduces a new ftrace handler that stores the registers on the stack before calling the next stage. The registers are restored from the stack before going back to the instrumented function.

A side-effect of this patch is to activate the support for ftrace_modify_call() as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture

Signed-off-by Jean-Jacques Hiblot <[email protected]>

Signed-off-by Abel Vesa <[email protected]>

---
 arch/arm/Kconfig               |  2 ++
 arch/arm/include/asm/ftrace.h  |  4 +++
 arch/arm/kernel/entry-ftrace.S | 78 ++++++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/ftrace.c       | 33 ++++++++++++++++++
 4 files changed, 117 insertions(+)

arch/arm/Kconfig +2 -0

index b5d529f..87f1a9f 100644
@@ -50,6 +50,7 @@ config ARM
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
+	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
 	select HAVE_EXIT_THREAD
 	select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)

@@ -90,6 +91,7 @@ config ARM
 	select PERF_USE_VMALLOC
 	select RTC_LIB
 	select SYS_SUPPORTS_APM_EMULATION
+	select FRAME_POINTER if DYNAMIC_FTRACE_WITH_REGS && FUNCTION_GRAPH_TRACER
 	# Above selects are sorted alphabetically; please add new ones
 	# according to that.  Thanks.
 	help

arch/arm/include/asm/ftrace.h +4 -0

index bfe2a2f..f434ce9 100644
@@ -1,6 +1,10 @@
 #ifndef _ASM_ARM_FTRACE
 #define _ASM_ARM_FTRACE
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+#define ARCH_SUPPORTS_FTRACE_OPS 1
+#endif
+
 #ifdef CONFIG_FUNCTION_TRACER
 #define MCOUNT_ADDR		((unsigned long)(__gnu_mcount_nc))
 #define MCOUNT_INSN_SIZE	4 /* sizeof mcount call */

arch/arm/kernel/entry-ftrace.S +78 -0

102 lines not on this page · raw message

arch/arm/kernel/ftrace.c +33 -0

index 3f17594..d8d4753 100644
@@ -139,6 +139,15 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
 
 	ret = ftrace_modify_code(pc, 0, new, false);
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+	if (!ret) {
+		pc = (unsigned long)&ftrace_regs_call;
+		new = ftrace_call_replace(pc, (unsigned long)func);
+
+		ret = ftrace_modify_code(pc, 0, new, false);
+	}
+#endif
+
 #ifdef CONFIG_OLD_MCOUNT
 	if (!ret) {
 		pc = (unsigned long)&ftrace_call_old;

@@ -157,6 +166,20 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	unsigned long ip = rec->ip;
 
 	old = ftrace_nop_replace(rec);
+
+	new = ftrace_call_replace(ip, adjust_address(rec, addr));
+
+	return ftrace_modify_code(rec->ip, old, new, true);
+}
+
+int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
+				unsigned long addr)
+{
+	unsigned long new, old;
+	unsigned long ip = rec->ip;
+
+	old = ftrace_call_replace(ip, adjust_address(rec, old_addr));
+
 	new = ftrace_call_replace(ip, adjust_address(rec, addr));
 
 	return ftrace_modify_code(rec->ip, old, new, true);

@@ -229,6 +252,8 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
 extern unsigned long ftrace_graph_call;
 extern unsigned long ftrace_graph_call_old;
 extern void ftrace_graph_caller_old(void);
+extern unsigned long ftrace_graph_regs_call;
+extern void ftrace_graph_regs_caller(void);
 
 static int __ftrace_modify_caller(unsigned long *callsite,
 				  void (*func) (void), bool enable)

@@ -251,6 +276,14 @@ static int ftrace_modify_graph_caller(bool enable)
 				     ftrace_graph_caller,
 				     enable);
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+	if (!ret)
+		ret = __ftrace_modify_caller(&ftrace_graph_regs_call,
+				     ftrace_graph_regs_caller,
+				     enable);
+#endif
+
+
 #ifdef CONFIG_OLD_MCOUNT
 	if (!ret)
 		ret = __ftrace_modify_caller(&ftrace_graph_call_old,

Signature · 1 line

2.7.4

Abel Vesa

2016-12-08 22:46 UTC #1 lore raw

Quoted 240 lines · Abel Vesa

From: Jean-Jacques Hiblot <[email protected]>

From statement twice in the commit message. Will resend.

A side-effect of this patch is to activate the support for ftrace_modify_call() as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture

Signed-off-by Jean-Jacques Hiblot <[email protected]>

Signed-off-by Abel Vesa <[email protected]>

---
 arch/arm/Kconfig               |  2 ++
 arch/arm/include/asm/ftrace.h  |  4 +++
 arch/arm/kernel/entry-ftrace.S | 78 ++++++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/ftrace.c       | 33 ++++++++++++++++++
 4 files changed, 117 insertions(+)

arch/arm/Kconfig +2 -0

index b5d529f..87f1a9f 100644
@@ -50,6 +50,7 @@ config ARM
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
+	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
 	select HAVE_EXIT_THREAD
 	select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)

@@ -90,6 +91,7 @@ config ARM
 	select PERF_USE_VMALLOC
 	select RTC_LIB
 	select SYS_SUPPORTS_APM_EMULATION
+	select FRAME_POINTER if DYNAMIC_FTRACE_WITH_REGS && FUNCTION_GRAPH_TRACER
 	# Above selects are sorted alphabetically; please add new ones
 	# according to that.  Thanks.
 	help

arch/arm/include/asm/ftrace.h +4 -0

index bfe2a2f..f434ce9 100644
@@ -1,6 +1,10 @@
 #ifndef _ASM_ARM_FTRACE
 #define _ASM_ARM_FTRACE
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+#define ARCH_SUPPORTS_FTRACE_OPS 1
+#endif
+
 #ifdef CONFIG_FUNCTION_TRACER
 #define MCOUNT_ADDR		((unsigned long)(__gnu_mcount_nc))
 #define MCOUNT_INSN_SIZE	4 /* sizeof mcount call */

arch/arm/kernel/entry-ftrace.S +78 -0

index c73c403..fd75bae 100644
@@ -92,12 +92,73 @@
 2:	mcount_exit
 .endm
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+
+.macro __ftrace_regs_caller
+
+	add 	ip, sp, #4	@ move in IP the value of SP as it was
+				@ before the push {lr} of the mcount mechanism
+	stmdb	sp!, {ip,lr,pc}
+	stmdb	sp!, {r0-r11,lr}
+
+	@ stack content at this point:
+	@ 0  4          44    48   52       56   60   64
+	@ RO | R1 | ... | R11 | LR | SP + 4 | LR | PC | previous LR |
+
+	mov r3, sp				@ struct pt_regs*
+	ldr r2, =function_trace_op
+	ldr r2, [r2]				@ pointer to the current
+						@ function tracing op
+	ldr	r1, [sp, #64]			@ lr of instrumented func
+	mcount_adjust_addr	r0, lr		@ instrumented function
+
+	.globl ftrace_regs_call
+ftrace_regs_call:
+	bl	ftrace_stub
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+	.globl ftrace_graph_regs_call
+ftrace_graph_regs_call:
+	mov	r0, r0
+#endif
+	ldr	lr, [sp, #64]		@ get the previous LR value from stack
+	ldmia	sp, {r0-r11, ip, sp}	@ pop the saved registers INCLUDING
+					@ the stack pointer
+	ret	ip
+.endm
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+.macro __ftrace_graph_regs_caller
+
+	sub	r0, fp, #4			@ lr of instrumented routine (parent)
+
+	@ called from __ftrace_regs_caller
+	ldr	r1, [sp, #56]			@ instrumented routine (func)
+	mcount_adjust_addr	r1, r1
+
+	mov	r2, fp				@ frame pointer
+	bl	prepare_ftrace_return
+
+	ldr	lr, [fp, #-4]			@ restore lr from the stack
+	ldmia	sp, {r0-r11, ip, sp}		@ restore r0 through sp
+	ret	ip
+.endm
+#endif
+#endif
+
 .macro __ftrace_caller suffix
 	mcount_enter
 
 	mcount_get_lr	r1			@ lr of instrumented func
 	mcount_adjust_addr	r0, lr		@ instrumented function
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+	ldr r2, =function_trace_op
+	ldr r2, [r2]				@ pointer to the current
+						@ function tracing op
+	mov r3, #0				@ regs is NULL
+#endif
+
 	.globl ftrace_call\suffix
 ftrace_call\suffix:
 	bl	ftrace_stub

@@ -212,6 +273,15 @@ UNWIND(.fnstart)
 	__ftrace_caller
 UNWIND(.fnend)
 ENDPROC(ftrace_caller)
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+ENTRY(ftrace_regs_caller)
+UNWIND(.fnstart)
+	__ftrace_regs_caller
+UNWIND(.fnend)
+ENDPROC(ftrace_regs_caller)
+#endif
+
 #endif
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER

@@ -220,6 +290,14 @@ UNWIND(.fnstart)
 	__ftrace_graph_caller
 UNWIND(.fnend)
 ENDPROC(ftrace_graph_caller)
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+ENTRY(ftrace_graph_regs_caller)
+UNWIND(.fnstart)
+	__ftrace_graph_regs_caller
+UNWIND(.fnend)
+ENDPROC(ftrace_graph_regs_caller)
+#endif
 #endif
 
 .purgem mcount_enter

arch/arm/kernel/ftrace.c +33 -0

index 3f17594..d8d4753 100644
@@ -139,6 +139,15 @@ int ftrace_update_ftrace_func(ftrace_func_t func)
 
 	ret = ftrace_modify_code(pc, 0, new, false);
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+	if (!ret) {
+		pc = (unsigned long)&ftrace_regs_call;
+		new = ftrace_call_replace(pc, (unsigned long)func);
+
+		ret = ftrace_modify_code(pc, 0, new, false);
+	}
+#endif
+
 #ifdef CONFIG_OLD_MCOUNT
 	if (!ret) {
 		pc = (unsigned long)&ftrace_call_old;

@@ -157,6 +166,20 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	unsigned long ip = rec->ip;
 
 	old = ftrace_nop_replace(rec);
+
+	new = ftrace_call_replace(ip, adjust_address(rec, addr));
+
+	return ftrace_modify_code(rec->ip, old, new, true);
+}
+
+int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
+				unsigned long addr)
+{
+	unsigned long new, old;
+	unsigned long ip = rec->ip;
+
+	old = ftrace_call_replace(ip, adjust_address(rec, old_addr));
+
 	new = ftrace_call_replace(ip, adjust_address(rec, addr));
 
 	return ftrace_modify_code(rec->ip, old, new, true);

@@ -229,6 +252,8 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
 extern unsigned long ftrace_graph_call;
 extern unsigned long ftrace_graph_call_old;
 extern void ftrace_graph_caller_old(void);
+extern unsigned long ftrace_graph_regs_call;
+extern void ftrace_graph_regs_caller(void);
 
 static int __ftrace_modify_caller(unsigned long *callsite,
 				  void (*func) (void), bool enable)

@@ -251,6 +276,14 @@ static int ftrace_modify_graph_caller(bool enable)
 				     ftrace_graph_caller,
 				     enable);
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+	if (!ret)
+		ret = __ftrace_modify_caller(&ftrace_graph_regs_call,
+				     ftrace_graph_regs_caller,
+				     enable);
+#endif
+
+
 #ifdef CONFIG_OLD_MCOUNT
 	if (!ret)
 		ret = __ftrace_modify_caller(&ftrace_graph_call_old,

Signature · 1 line

2.7.4

Petr Mladek

2017-01-10 15:51 UTC #2 lore raw

Quoted 38 lines · Abel Vesa

Quoted 37 lines · Abel Vesa
From: Jean-Jacques Hiblot <[email protected]>
From: Jean-Jacques Hiblot <[email protected]>
From statement twice in the commit message. Will resend.
The DYNAMIC_FTRACE_WITH_REGS configuration makes it possible for a ftrace operation to specify if registers need to saved/restored by the ftrace handler. This is needed by kgraft and possibly other ftrace-based tools, and the ARM architecture is currently lacking this feature. It would also be the first step to support the "Kprobes-on-ftrace" optimization on ARM.
This patch introduces a new ftrace handler that stores the registers on the stack before calling the next stage. The registers are restored from the stack before going back to the instrumented function.
A side-effect of this patch is to activate the support for ftrace_modify_call() as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture
Signed-off-by Jean-Jacques Hiblot <[email protected]>
Signed-off-by Abel Vesa <[email protected]>
---
 arch/arm/Kconfig               |  2 ++
 arch/arm/include/asm/ftrace.h  |  4 +++
 arch/arm/kernel/entry-ftrace.S | 78 ++++++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/ftrace.c       | 33 ++++++++++++++++++
 4 files changed, 117 insertions(+)
arch/arm/Kconfig +1 -0
index b5d529f..87f1a9f 100644
@@ -50,6 +50,7 @@ config ARM
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
+	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE

Quoted 8 lines

arch/arm/Kconfig config ARM · line 94

Quoted 8 lines

arch/arm/Kconfig +1 -0

 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
 	select HAVE_EXIT_THREAD
 	select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)

@@ -90,6 +91,7 @@ config ARM
 	select PERF_USE_VMALLOC
 	select RTC_LIB
 	select SYS_SUPPORTS_APM_EMULATION
+	select FRAME_POINTER if DYNAMIC_FTRACE_WITH_REGS && FUNCTION_GRAPH_TRACER

FRAME_POINTER is not for free. It takes space on the stack. Also there is a performance penalty. Do we really need to depend on it? If so, it might be worth a note in the commit message.

I made only a quick look at the patch. It looks reasonable. But I do not have enough knowledge about the arm architecture, assembly, and ftrace-specifics. Also I cannot test it easily. So issues might be hidden to my eyes.

Best Regards,
Petr

Abel Vesa

2017-01-12 00:19 UTC #3 lore raw

Quoted 39 lines · Petr Mladek

Quoted 38 lines · Abel Vesa
Quoted 37 lines · Abel Vesa
From: Jean-Jacques Hiblot <[email protected]>
From: Jean-Jacques Hiblot <[email protected]>
From statement twice in the commit message. Will resend.
The DYNAMIC_FTRACE_WITH_REGS configuration makes it possible for a ftrace operation to specify if registers need to saved/restored by the ftrace handler. This is needed by kgraft and possibly other ftrace-based tools, and the ARM architecture is currently lacking this feature. It would also be the first step to support the "Kprobes-on-ftrace" optimization on ARM.
This patch introduces a new ftrace handler that stores the registers on the stack before calling the next stage. The registers are restored from the stack before going back to the instrumented function.
A side-effect of this patch is to activate the support for ftrace_modify_call() as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture
Signed-off-by Jean-Jacques Hiblot <[email protected]>
Signed-off-by Abel Vesa <[email protected]>
---
 arch/arm/Kconfig               |  2 ++
 arch/arm/include/asm/ftrace.h  |  4 +++
 arch/arm/kernel/entry-ftrace.S | 78 ++++++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/ftrace.c       | 33 ++++++++++++++++++
 4 files changed, 117 insertions(+)
arch/arm/Kconfig +1 -0
index b5d529f..87f1a9f 100644
@@ -50,6 +50,7 @@ config ARM
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
+	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE

Quoted 8 lines

arch/arm/Kconfig config ARM · line 94

Quoted 8 lines

arch/arm/Kconfig +1 -0

 	select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
 	select HAVE_EXIT_THREAD
 	select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)

@@ -90,6 +91,7 @@ config ARM
 	select PERF_USE_VMALLOC
 	select RTC_LIB
 	select SYS_SUPPORTS_APM_EMULATION
+	select FRAME_POINTER if DYNAMIC_FTRACE_WITH_REGS && FUNCTION_GRAPH_TRACER

Hi Petr,

Quoted 4 lines

FRAME_POINTER is not for free. It takes space on the stack. Also there is a performance penalty. Do we really need to depend on it? If so, it might be worth a note in the commit message.

I was trying to create my own patch when I found this work done by Jean-Jacques, so I haven't looked specifically for the FRAME_POINTER part. I looked now at it and you seem to be right, FRAME_POINTER is not needed.

I will get rid of the FRAME_POINTER part, change the authorship and send it again in the following days.

Quoted 8 lines

I made only a quick look at the patch. It looks reasonable. But I do not have enough knowledge about the arm architecture, assembly, and ftrace-specifics. Also I cannot test it easily. So issues might be hidden to my eyes.
Best Regards,
Petr

Thanks,
Abel

Jean-Jacques Hiblot

2017-01-12 14:30 UTC #4 lore raw

2017-01-12 1:19 GMT+01:00 Abel Vesa <[email protected]>:

Quoted 41 lines · Petr Mladek

Quoted 40 lines · Abel Vesa
Quoted 39 lines · Abel Vesa
Quoted 38 lines
From: Jean-Jacques Hiblot <[email protected]>
From: Jean-Jacques Hiblot <[email protected]>
From statement twice in the commit message. Will resend.
The DYNAMIC_FTRACE_WITH_REGS configuration makes it possible for a ftrace operation to specify if registers need to saved/restored by the ftrace handler. This is needed by kgraft and possibly other ftrace-based tools, and the ARM architecture is currently lacking this feature. It would also be the first step to support the "Kprobes-on-ftrace" optimization on ARM.
This patch introduces a new ftrace handler that stores the registers on the stack before calling the next stage. The registers are restored from the stack before going back to the instrumented function.
A side-effect of this patch is to activate the support for ftrace_modify_call() as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture
Signed-off-by Jean-Jacques Hiblot <[email protected]>
Signed-off-by Abel Vesa <[email protected]>
---
 arch/arm/Kconfig               |  2 ++
 arch/arm/include/asm/ftrace.h  |  4 +++
 arch/arm/kernel/entry-ftrace.S | 78 ++++++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/ftrace.c       | 33 ++++++++++++++++++
 4 files changed, 117 insertions(+)
arch/arm/Kconfig +1 -0
index b5d529f..87f1a9f 100644
@@ -50,6 +50,7 @@ config ARM
  select HAVE_DMA_API_DEBUG
  select HAVE_DMA_CONTIGUOUS if MMU
  select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
+ select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
  select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU

Quoted 13 lines

Quoted 7 lines
Quoted 7 lines
arch/arm/Kconfig config ARM · line 94
Quoted 7 lines
arch/arm/Kconfig +1 -0
  select HAVE_EXIT_THREAD
  select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)
@@ -90,6 +91,7 @@ config ARM
  select PERF_USE_VMALLOC
  select RTC_LIB
  select SYS_SUPPORTS_APM_EMULATION
+ select FRAME_POINTER if DYNAMIC_FTRACE_WITH_REGS && FUNCTION_GRAPH_TRACER
Hi Petr,
Quoted 4 lines
FRAME_POINTER is not for free. It takes space on the stack. Also there is a performance penalty. Do we really need to depend on it? If so, it might be worth a note in the commit message.

FRAME_POINTER is not needed. the dependency is wrong and should be removed. The code must be modified to not use fp register:

arch/arm/kernel/entry-ftrace.S +0 -1

@@ -130,7 +130,8 @@ ftrace_graph_regs_call:
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 .macro __ftrace_graph_regs_caller
 
-       sub     r0, fp, #4                      @ lr of instrumented

routine (parent)
+ add r0, sp, #64 @ r0 is now a pointer to lr of
+ @ instrumented routine

@ called from __ftrace_regs_caller
ldr r1, [sp, #56] @ instrumented routine (func)
@@ -139,8 +140,9 @@ ftrace_graph_regs_call:
mov r2, fp @ frame pointer
bl prepare_ftrace_return

-       ldr     lr, [fp, #-4]                   @ restore lr from the stack
-       ldmia   sp, {r0-r11, ip, sp}            @ restore r0 through sp
+       ldr     lr, [sp, #64]           @ get the previous LR value from stack
+       ldmia   sp, {r0-r11, ip, sp}    @ pop the saved registers INCLUDING

+ @ the stack pointer
ret ip
.endm
#endif

Jean-Jacques

Quoted 17 lines

I was trying to create my own patch when I found this work done by Jean-Jacques, so I haven't looked specifically for the FRAME_POINTER part. I looked now at it and you seem to be right, FRAME_POINTER is not needed.
I will get rid of the FRAME_POINTER part, change the authorship and send it again in the following days.
Quoted 8 lines
I made only a quick look at the patch. It looks reasonable. But I do not have enough knowledge about the arm architecture, assembly, and ftrace-specifics. Also I cannot test it easily. So issues might be hidden to my eyes.
Best Regards,
Petr
Thanks,
Abel

Jean-Jacques Hiblot

2017-01-13 08:30 UTC #5 lore raw

2017-01-12 15:30 GMT+01:00 Jean-Jacques Hiblot <[email protected]>:

Quoted 62 lines

2017-01-12 1:19 GMT+01:00 Abel Vesa <[email protected]>:
Quoted 41 lines · Petr Mladek
Quoted 40 lines · Abel Vesa
Quoted 39 lines · Abel Vesa
Quoted 38 lines
From: Jean-Jacques Hiblot <[email protected]>
From: Jean-Jacques Hiblot <[email protected]>
From statement twice in the commit message. Will resend.
The DYNAMIC_FTRACE_WITH_REGS configuration makes it possible for a ftrace operation to specify if registers need to saved/restored by the ftrace handler. This is needed by kgraft and possibly other ftrace-based tools, and the ARM architecture is currently lacking this feature. It would also be the first step to support the "Kprobes-on-ftrace" optimization on ARM.
This patch introduces a new ftrace handler that stores the registers on the stack before calling the next stage. The registers are restored from the stack before going back to the instrumented function.
A side-effect of this patch is to activate the support for ftrace_modify_call() as it defines ARCH_SUPPORTS_FTRACE_OPS for the ARM architecture
Signed-off-by Jean-Jacques Hiblot <[email protected]>
Signed-off-by Abel Vesa <[email protected]>
---
 arch/arm/Kconfig               |  2 ++
 arch/arm/include/asm/ftrace.h  |  4 +++
 arch/arm/kernel/entry-ftrace.S | 78 ++++++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/ftrace.c       | 33 ++++++++++++++++++
 4 files changed, 117 insertions(+)
arch/arm/Kconfig +1 -0
index b5d529f..87f1a9f 100644
@@ -50,6 +50,7 @@ config ARM
  select HAVE_DMA_API_DEBUG
  select HAVE_DMA_CONTIGUOUS if MMU
  select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU
+ select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
  select HAVE_EFFICIENT_UNALIGNED_ACCESS if (CPU_V6 || CPU_V6K || CPU_V7) && MMU
Quoted 13 lines
Quoted 7 lines
Quoted 7 lines
arch/arm/Kconfig config ARM · line 94
Quoted 7 lines
arch/arm/Kconfig +1 -0
  select HAVE_EXIT_THREAD
  select HAVE_FTRACE_MCOUNT_RECORD if (!XIP_KERNEL)
@@ -90,6 +91,7 @@ config ARM
  select PERF_USE_VMALLOC
  select RTC_LIB
  select SYS_SUPPORTS_APM_EMULATION
+ select FRAME_POINTER if DYNAMIC_FTRACE_WITH_REGS && FUNCTION_GRAPH_TRACER
Hi Petr,
Quoted 4 lines
FRAME_POINTER is not for free. It takes space on the stack. Also there is a performance penalty. Do we really need to depend on it? If so, it might be worth a note in the commit message.
FRAME_POINTER is not needed. the dependency is wrong and should be removed. The code must be modified to not use fp register:

arch/arm/kernel/entry-ftrace.S ftrace_graph_regs_call: · line 135

Quoted 7 lines

arch/arm/kernel/entry-ftrace.S +2 -1

@@ -130,7 +130,8 @@ ftrace_graph_regs_call:
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 .macro __ftrace_graph_regs_caller
 
-       sub     r0, fp, #4                      @ lr of instrumented
 routine (parent)
+       add     r0, sp, #64             @ r0 is now a pointer to lr of
+                                       @ instrumented routine

I made some tests after sending this email. And it turns out that it
doesn't work if we change "sub r0, fp, #4" to "add r0, sp,
#64 " here.
So it looks like there is a dependency on FRAME_POINTER after all. Note that the same is true for __ftrace_graph_caller. I don't know if the 'graph' feature of ftrace requires intrinsically FRAME_POINTER but it looks like it currently does on ARM (with or without register saving)
I'll try to spend some time on the subject next week.

Quoted 37 lines

arch/arm/kernel/entry-ftrace.S +3 -2
        @ called from __ftrace_regs_caller
        ldr     r1, [sp, #56]                   @ instrumented routine (func)
@@ -139,8 +140,9 @@ ftrace_graph_regs_call:
        mov     r2, fp                          @ frame pointer
        bl      prepare_ftrace_return
 
-       ldr     lr, [fp, #-4]                   @ restore lr from the stack
-       ldmia   sp, {r0-r11, ip, sp}            @ restore r0 through sp
+       ldr     lr, [sp, #64]           @ get the previous LR value from stack
+       ldmia   sp, {r0-r11, ip, sp}    @ pop the saved registers INCLUDING
+                                       @ the stack pointer
        ret     ip
 .endm
 #endif
Jean-Jacques
Quoted 17 lines
I was trying to create my own patch when I found this work done by Jean-Jacques, so I haven't looked specifically for the FRAME_POINTER part. I looked now at it and you seem to be right, FRAME_POINTER is not needed.
I will get rid of the FRAME_POINTER part, change the authorship and send it again in the following days.
Quoted 8 lines
I made only a quick look at the patch. It looks reasonable. But I do not have enough knowledge about the arm architecture, assembly, and ftrace-specifics. Also I cannot test it easily. So issues might be hidden to my eyes.
Best Regards,
Petr
Thanks,
Abel

Abel Vesa

2017-01-24 00:43 UTC #6 lore raw

Hi Jean-Jacques,

Here is the implementation I've made for ftrace_graph_regs_caller:

.macro __ftrace_graph_regs_caller

sub r0, fp, #4 @ lr of
instrumented routine (parent)

@ called from __ftrace_regs_caller
ldr r1, [sp, #56] @ instrumented routine (func)
mcount_adjust_addr r1, r1

sub r2, r0, #4 @ frame pointer
bl prepare_ftrace_return

ldr lr, [sp, #64] @ restore lr from the stack
ldmia sp, {r0-r11, ip, sp} @ restore r0 through sp
ret ip

.endm

AFAIK, you can still use fp (see the implementation without regs) since it is an alternative name for r11. The FRAME_POINTER config options does something else entirely and has nothing to do with what we need here.

I tested it but unfortunately, my current setup is with qemu and I don't have a real hardware here close to test it properly. Could you please tell me if this works on your setup?

Also the way I tested it was to comment out the code that uses the default ftrace_graph_caller in ftrace_modify_graph_caller and enforced the usage of the one with regs.

I will create a proper patch later today (I need to cleanup some stuff first) and
send it.

Thanks,
Abel