Re: [RFC PATCH 4/4] ftrace/x86: Add support for -mfentry to x86_64

[Steven Rostedt <rostedt@goodmis.org>]

Peter and Masami

During my final tests, I found that this change breaks the
!DYNAMIC_FTRACE config. That is, when we don't do the run-time updates
of mcount calls to nops, the compiler will use fentry but the code still
uses mcount.

I fixed this in the patch below. But as you two have acked and reviewed
it, I can't add your tags if I have changed the code. Can you ack/review
it again.

Thanks!

-- Steve


On Tue, 2012-08-07 at 15:38 -0400, Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@redhat.com>
> 
> If the kernel is compiled with gcc 4.6.0 which supports -mfentry,
> then use that instead of mcount.
> 
> With mcount, frame pointers are forced with the -pg option and we
> get something like:
> 
> <can_vma_merge_before>:
>        55                      push   %rbp
>        48 89 e5                mov    %rsp,%rbp
>        53                      push   %rbx
>        41 51                   push   %r9
>        e8 fe 6a 39 00          callq  ffffffff81483d00 <mcount>
>        31 c0                   xor    %eax,%eax
>        48 89 fb                mov    %rdi,%rbx
>        48 89 d7                mov    %rdx,%rdi
>        48 33 73 30             xor    0x30(%rbx),%rsi
>        48 f7 c6 ff ff ff f7    test   $0xfffffffff7ffffff,%rsi
> 
> With -mfentry, frame pointers are no longer forced and the call looks
> like this:
> 
> <can_vma_merge_before>:
>        e8 33 af 37 00          callq  ffffffff81461b40 <__fentry__>
>        53                      push   %rbx
>        48 89 fb                mov    %rdi,%rbx
>        31 c0                   xor    %eax,%eax
>        48 89 d7                mov    %rdx,%rdi
>        41 51                   push   %r9
>        48 33 73 30             xor    0x30(%rbx),%rsi
>        48 f7 c6 ff ff ff f7    test   $0xfffffffff7ffffff,%rsi
> 
> This adds the ftrace hook at the beginning of the function before a
> frame is set up, and allows the function callbacks to be able to access
> parameters. As kprobes now can use function tracing (at least on x86)
> this speeds up the kprobe hooks that are at the beginning of the
> function.
> 
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> Cc: Andi Kleen <andi@firstfloor.org>
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

(change log kept the same)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c70684f..bbbf5d8 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -36,6 +36,7 @@ config X86
 	select HAVE_KRETPROBES
 	select HAVE_OPTPROBES
 	select HAVE_FTRACE_MCOUNT_RECORD
+	select HAVE_FENTRY if X86_64
 	select HAVE_C_RECORDMCOUNT
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_FUNCTION_TRACER
diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
index a6cae0c..9a25b52 100644
--- a/arch/x86/include/asm/ftrace.h
+++ b/arch/x86/include/asm/ftrace.h
@@ -35,7 +35,11 @@
 #endif
 
 #ifdef CONFIG_FUNCTION_TRACER
-#define MCOUNT_ADDR		((long)(mcount))
+#ifdef CC_USING_FENTRY
+# define MCOUNT_ADDR		((long)(__fentry__))
+#else
+# define MCOUNT_ADDR		((long)(mcount))
+#endif
 #define MCOUNT_INSN_SIZE	5 /* sizeof mcount call */
 
 #ifdef CONFIG_DYNAMIC_FTRACE
@@ -46,6 +50,7 @@
 #ifndef __ASSEMBLY__
 extern void mcount(void);
 extern atomic_t modifying_ftrace_code;
+extern void __fentry__(void);
 
 static inline unsigned long ftrace_call_adjust(unsigned long addr)
 {
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 38308fa..a698521 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -68,10 +68,18 @@
 	.section .entry.text, "ax"
 
 #ifdef CONFIG_FUNCTION_TRACER
+
+#ifdef CC_USING_FENTRY
+# define function_hook	__fentry__
+#else
+# define function_hook	mcount
+#endif
+
 #ifdef CONFIG_DYNAMIC_FTRACE
-ENTRY(mcount)
+
+ENTRY(function_hook)
 	retq
-END(mcount)
+END(function_hook)
 
 /* skip is set if stack has been adjusted */
 .macro ftrace_caller_setup skip=0
@@ -84,7 +92,11 @@ END(mcount)
 	movq RIP(%rsp), %rdi
 	subq $MCOUNT_INSN_SIZE, %rdi
 	/* Load the parent_ip into the second parameter */
+#ifdef CC_USING_FENTRY
+	movq SS+16(%rsp), %rsi
+#else
 	movq 8(%rbp), %rsi
+#endif
 .endm
 
 ENTRY(ftrace_caller)
@@ -177,7 +189,8 @@ END(ftrace_regs_caller)
 
 
 #else /* ! CONFIG_DYNAMIC_FTRACE */
-ENTRY(mcount)
+
+ENTRY(function_hook)
 	cmpl $0, function_trace_stop
 	jne  ftrace_stub
 
@@ -199,7 +212,11 @@ trace:
 	MCOUNT_SAVE_FRAME
 
 	movq RIP(%rsp), %rdi
+#ifdef CC_USING_FENTRY
+	movq SS+16(%rsp), %rsi
+#else
 	movq 8(%rbp), %rsi
+#endif
 	subq $MCOUNT_INSN_SIZE, %rdi
 
 	call   *ftrace_trace_function
@@ -207,7 +224,7 @@ trace:
 	MCOUNT_RESTORE_FRAME
 
 	jmp ftrace_stub
-END(mcount)
+END(function_hook)
 #endif /* CONFIG_DYNAMIC_FTRACE */
 #endif /* CONFIG_FUNCTION_TRACER */
 
@@ -215,9 +232,14 @@ END(mcount)
 ENTRY(ftrace_graph_caller)
 	MCOUNT_SAVE_FRAME
 
+#ifdef CC_USING_FENTRY
+	leaq SS+16(%rsp), %rdi
+	movq $0, %rdx	/* No framepointers needed */
+#else
 	leaq 8(%rbp), %rdi
-	movq RIP(%rsp), %rsi
 	movq (%rbp), %rdx
+#endif
+	movq RIP(%rsp), %rsi
 	subq $MCOUNT_INSN_SIZE, %rsi
 
 	call	prepare_ftrace_return
diff --git a/arch/x86/kernel/x8664_ksyms_64.c b/arch/x86/kernel/x8664_ksyms_64.c
index 9796c2f..643b236 100644
--- a/arch/x86/kernel/x8664_ksyms_64.c
+++ b/arch/x86/kernel/x8664_ksyms_64.c
@@ -13,9 +13,13 @@
 #include <asm/ftrace.h>
 
 #ifdef CONFIG_FUNCTION_TRACER
-/* mcount is defined in assembly */
+/* mcount and __fentry__ are defined in assembly */
+#ifdef CC_USING_FENTRY
+EXPORT_SYMBOL(__fentry__);
+#else
 EXPORT_SYMBOL(mcount);
 #endif
+#endif
 
 EXPORT_SYMBOL(__get_user_1);
 EXPORT_SYMBOL(__get_user_2);