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Commit 562e14f7 authored by Steven Rostedt (VMware)'s avatar Steven Rostedt (VMware)
Browse files

ftrace/x86: Remove mcount support 



There's two methods of enabling function tracing in Linux on x86. One is
with just "gcc -pg" and the other is "gcc -pg -mfentry". The former will use
calls to a special function "mcount" after the frame is set up in all C
functions. The latter will add calls to a special function called "fentry"
as the very first instruction of all C functions.

At compile time, there is a check to see if gcc supports, -mfentry, and if
it does, it will use that, because it is more versatile and less error prone
for function tracing.

Starting with v4.19, the minimum gcc supported to build the Linux kernel,
was raised to version 4.6. That also happens to be the first gcc version to
support -mfentry. Since on x86, using gcc versions from 4.6 and beyond will
unconditionally enable the -mfentry, it will no longer use mcount as the
method for inserting calls into the C functions of the kernel. This means
that there is no point in continuing to maintain mcount in x86.

Remove support for using mcount. This makes the code less complex, and will
also allow it to be simplified in the future.

Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarJiri Kosina <jkosina@suse.cz>
Acked-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
parent 518049d9

arch/x86/include/asm/ftrace.h

+3 −5
Original line number Diff line number Diff line
@@ -3,12 +3,10 @@ 
#define _ASM_X86_FTRACE_H



#ifdef CONFIG_FUNCTION_TRACER

#ifdef CC_USING_FENTRY

# define MCOUNT_ADDR		((unsigned long)(__fentry__))

#else

# define MCOUNT_ADDR		((unsigned long)(mcount))

# define HAVE_FUNCTION_GRAPH_FP_TEST

#ifndef CC_USING_FENTRY

# error Compiler does not support fentry?

#endif

# define MCOUNT_ADDR		((unsigned long)(__fentry__))

#define MCOUNT_INSN_SIZE	5 /* sizeof mcount call */



#ifdef CONFIG_DYNAMIC_FTRACE

arch/x86/include/asm/livepatch.h

+0 −3
Original line number Diff line number Diff line
@@ -26,9 +26,6 @@ 


static inline int klp_check_compiler_support(void)

{

#ifndef CC_USING_FENTRY

	return 1;

#endif

	return 0;

}

arch/x86/kernel/ftrace_32.S

+5 −31
Original line number Diff line number Diff line
@@ -10,20 +10,10 @@ 
#include <asm/ftrace.h>

#include <asm/nospec-branch.h>



#ifdef CC_USING_FENTRY

# define function_hook	__fentry__

EXPORT_SYMBOL(__fentry__)

#else

# define function_hook	mcount

EXPORT_SYMBOL(mcount)

#endif



/* 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

#ifdef CONFIG_FRAME_POINTER

# define MCOUNT_FRAME			1	/* using frame = true  */

#else

# define MCOUNT_FRAME			0	/* using frame = false */
@@ -35,8 +25,7 @@ END(function_hook) 


ENTRY(ftrace_caller)



#ifdef USING_FRAME_POINTER

# ifdef CC_USING_FENTRY

#ifdef CONFIG_FRAME_POINTER

	/*

	 * Frame pointers are of ip followed by bp.

	 * Since fentry is an immediate jump, we are left with
@@ -47,7 +36,7 @@ ENTRY(ftrace_caller) 
	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
@@ -57,7 +46,7 @@ ENTRY(ftrace_caller) 
	pushl	%edx

	pushl	$0				/* Pass NULL as regs pointer */



#ifdef USING_FRAME_POINTER

#ifdef CONFIG_FRAME_POINTER

	/* Load parent ebp into edx */

	movl	4*4(%esp), %edx

#else
@@ -80,14 +69,12 @@ ftrace_call: 
	popl	%edx

	popl	%ecx

	popl	%eax

#ifdef USING_FRAME_POINTER

#ifdef CONFIG_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
@@ -131,11 +118,7 @@ ENTRY(ftrace_regs_caller) 


	movl	12*4(%esp), %eax		/* Load ip (1st parameter) */

	subl	$MCOUNT_INSN_SIZE, %eax		/* Adjust ip */

#ifdef CC_USING_FENTRY

	movl	15*4(%esp), %edx		/* Load parent ip (2nd parameter) */

#else

	movl	0x4(%ebp), %edx			/* Load parent ip (2nd parameter) */

#endif

	movl	function_trace_op, %ecx		/* Save ftrace_pos in 3rd parameter */

	pushl	%esp				/* Save pt_regs as 4th parameter */
@@ -176,13 +159,8 @@ ENTRY(ftrace_graph_caller) 
	pushl	%edx

	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
@@ -195,11 +173,7 @@ 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

arch/x86/kernel/ftrace_64.S

+1 −27
Original line number Diff line number Diff line
@@ -13,23 +13,13 @@ 
	.code64

	.section .entry.text, "ax"



#ifdef CC_USING_FENTRY

# define function_hook	__fentry__

EXPORT_SYMBOL(__fentry__)

#else

# define function_hook	mcount

EXPORT_SYMBOL(mcount)

#endif



#ifdef CONFIG_FRAME_POINTER

# ifdef CC_USING_FENTRY

/* Save parent and function stack frames (rip and rbp) */

#  define MCOUNT_FRAME_SIZE	(8+16*2)

#else

/* Save just function stack frame (rip and rbp) */

#  define MCOUNT_FRAME_SIZE	(8+16)

# endif

#else

/* No need to save a stack frame */

# define MCOUNT_FRAME_SIZE	0

#endif /* CONFIG_FRAME_POINTER */
@@ -75,17 +65,13 @@ EXPORT_SYMBOL(mcount) 
	 * fentry is called before the stack frame is set up, where as mcount

	 * is called afterward.

	 */

#ifdef CC_USING_FENTRY



	/* Save the parent pointer (skip orig rbp and our return address) */

	pushq \added+8*2(%rsp)

	pushq %rbp

	movq %rsp, %rbp

	/* Save the return address (now skip orig rbp, rbp and parent) */

	pushq \added+8*3(%rsp)

#else

	/* Can't assume that rip is before this (unless added was zero) */

	pushq \added+8(%rsp)

#endif

	pushq %rbp

	movq %rsp, %rbp

#endif /* CONFIG_FRAME_POINTER */
@@ -113,12 +99,7 @@ EXPORT_SYMBOL(mcount) 
	movq %rdx, RBP(%rsp)



	/* Copy the parent address into %rsi (second parameter) */

#ifdef CC_USING_FENTRY

	movq MCOUNT_REG_SIZE+8+\added(%rsp), %rsi

#else

	/* %rdx contains original %rbp */

	movq 8(%rdx), %rsi

#endif



	 /* Move RIP to its proper location */

	movq MCOUNT_REG_SIZE+\added(%rsp), %rdi
@@ -303,15 +284,8 @@ ENTRY(ftrace_graph_caller) 
	/* Saves rbp into %rdx and fills first parameter  */

	save_mcount_regs



#ifdef CC_USING_FENTRY

	leaq MCOUNT_REG_SIZE+8(%rsp), %rsi

	movq $0, %rdx	/* No framepointers needed */

#else

	/* Save address of the return address of traced function */

	leaq 8(%rdx), %rsi

	/* ftrace does sanity checks against frame pointers */

	movq (%rdx), %rdx

#endif

	call	prepare_ftrace_return



	restore_mcount_regs