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Subject[PATCH 5/6 v3] ftrace/x86_32: Add -mfentry support to x86_32 with DYNAMIC_FTRACE set
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

x86_64 has had fentry support for some time. I did not add support to x86_32
as I was unsure if it will be used much in the future. It is still very much
used, and there's issues with function graph tracing with gcc playing around
with the mcount frames, causing function graph to panic. The fentry code
does not have this issue, and is able to cope as there is no frame to mess
up.

Note, this only add support for fentry when DYNAMIC_FTRACE is set. There's
really no reason to not have that set, because the performance of the
machine drops significantly when it's not enabled. I only keep
!DYNAMIC_FTRACE around to test it off, as there's still some archs that have
FTRACE but not DYNAMIC_FTRACE.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
arch/x86/Kconfig | 2 +-
arch/x86/kernel/ftrace_32.S | 82 +++++++++++++++++++++++++++++++++++++++------
2 files changed, 73 insertions(+), 11 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index cc98d5a294ee..8c17146427ca 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -127,7 +127,7 @@ config X86
select HAVE_EBPF_JIT if X86_64
select HAVE_EFFICIENT_UNALIGNED_ACCESS
select HAVE_EXIT_THREAD
- select HAVE_FENTRY if X86_64
+ select HAVE_FENTRY if X86_64 || DYNAMIC_FTRACE
select HAVE_FTRACE_MCOUNT_RECORD
select HAVE_FUNCTION_GRAPH_TRACER
select HAVE_FUNCTION_TRACER
diff --git a/arch/x86/kernel/ftrace_32.S b/arch/x86/kernel/ftrace_32.S
index 7cd27cf56578..fe64fa596266 100644
--- a/arch/x86/kernel/ftrace_32.S
+++ b/arch/x86/kernel/ftrace_32.S
@@ -11,26 +11,68 @@
#include <asm/ftrace.h>

#ifdef CONFIG_FUNCTION_TRACER
+
+#ifdef CC_USING_FENTRY
+# define function_hook __fentry__
+EXPORT_SYMBOL(__fentry__)
+#else
+# define function_hook mcount
+EXPORT_SYMBOL(mcount)
+#endif
+
#ifdef CONFIG_DYNAMIC_FTRACE

-ENTRY(mcount)
+/* mcount uses a frame pointer even if CONFIG_FRAME_POINTER is not set */
+#if !defined(CC_USING_FENTRY) || defined(CONFIG_FRAME_POINTER)
+# define USING_FRAME_POINTER
+#endif
+
+#ifdef USING_FRAME_POINTER
+# define MCOUNT_FRAME 1 /* using frame = true */
+#else
+# define MCOUNT_FRAME 0 /* using frame = false */
+#endif
+
+ENTRY(function_hook)
ret
-END(mcount)
+END(function_hook)

ENTRY(ftrace_caller)

+#ifdef USING_FRAME_POINTER
+# ifdef CC_USING_FENTRY
+ /*
+ * Frame pointers are of ip followed by bp.
+ * Since fentry is an immediate jump, we are left with
+ * parent-ip, function-ip. We need to add a frame with
+ * parent-ip followed by ebp.
+ */
+ pushl 4(%esp) /* parent ip */
pushl %ebp
movl %esp, %ebp
-
+ pushl 2*4(%esp) /* function ip */
+# endif
+ /* For mcount, the function ip is directly above */
+ pushl %ebp
+ movl %esp, %ebp
+#endif
pushl %eax
pushl %ecx
pushl %edx
pushl $0 /* Pass NULL as regs pointer */
- movl 5*4(%esp), %eax
- /* Copy original ebp into %edx */
+
+#ifdef USING_FRAME_POINTER
+ /* Load parent ebp into edx */
movl 4*4(%esp), %edx
+#else
+ /* There's no frame pointer, load the appropriate stack addr instead */
+ lea 4*4(%esp), %edx
+#endif
+
+ movl (MCOUNT_FRAME+4)*4(%esp), %eax /* load the rip */
/* Get the parent ip */
- movl 0x4(%edx), %edx
+ movl 4(%edx), %edx /* edx has ebp */
+
movl function_trace_op, %ecx
subl $MCOUNT_INSN_SIZE, %eax

@@ -42,7 +84,14 @@ ftrace_call:
popl %edx
popl %ecx
popl %eax
+#ifdef USING_FRAME_POINTER
popl %ebp
+# ifdef CC_USING_FENTRY
+ addl $4,%esp /* skip function ip */
+ popl %ebp /* this is the orig bp */
+ addl $4, %esp /* skip parent ip */
+# endif
+#endif
.Lftrace_ret:
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
.globl ftrace_graph_call
@@ -83,6 +132,10 @@ ENTRY(ftrace_regs_caller)
pushl %edx
pushl %ecx
pushl %ebx
+#ifdef CC_USING_FENTRY
+ /* Load 4 off of the parent ip addr into ebp */
+ lea 14*4(%esp), %ebp
+#endif

movl 12*4(%esp), %eax /* Load ip (1st parameter) */
subl $MCOUNT_INSN_SIZE, %eax /* Adjust ip */
@@ -121,7 +174,7 @@ GLOBAL(ftrace_regs_call)
jmp .Lftrace_ret
#else /* ! CONFIG_DYNAMIC_FTRACE */

-ENTRY(mcount)
+ENTRY(function_hook)
cmpl $__PAGE_OFFSET, %esp
jb ftrace_stub /* Paging not enabled yet? */

@@ -153,9 +206,8 @@ ftrace_stub:
popl %ecx
popl %eax
jmp ftrace_stub
-END(mcount)
+END(function_hook)
#endif /* CONFIG_DYNAMIC_FTRACE */
-EXPORT_SYMBOL(mcount)
#endif /* CONFIG_FUNCTION_TRACER */

#ifdef CONFIG_FUNCTION_GRAPH_TRACER
@@ -163,9 +215,15 @@ ENTRY(ftrace_graph_caller)
pushl %eax
pushl %ecx
pushl %edx
- movl 0xc(%esp), %eax
+ movl 3*4(%esp), %eax
+ /* Even with frame pointers, fentry doesn't have one here */
+#ifdef CC_USING_FENTRY
+ lea 4*4(%esp), %edx
+ movl $0, %ecx
+#else
lea 0x4(%ebp), %edx
movl (%ebp), %ecx
+#endif
subl $MCOUNT_INSN_SIZE, %eax
call prepare_ftrace_return
popl %edx
@@ -178,7 +236,11 @@ END(ftrace_graph_caller)
return_to_handler:
pushl %eax
pushl %edx
+#ifdef CC_USING_FENTRY
+ movl $0, %eax
+#else
movl %ebp, %eax
+#endif
call ftrace_return_to_handler
movl %eax, %ecx
popl %edx
--
2.10.2

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