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Subject[for-next][PATCH 2/5] ftrace/x86_32: Remove support for non DYNAMIC_FTRACE
From: "Steven Rostedt (VMware)" <>

When DYNAMIC_FTRACE is enabled in the kernel, all the functions that can be
traced by the function tracer have a "nop" placeholder at the start of the
function. When function tracing is enabled, the nop is converted into a call
to the tracing infrastructure where the functions get traced. This also
allows for specifying specific functions to trace, and a lot of
infrastructure is built on top of this.

When DYNAMIC_FTRACE is not enabled, all the functions have a call to the
ftrace trampoline. A check is made to see if a function pointer is the
ftrace_stub or not, and if it is not, it calls the function pointer to trace
the code. This adds over 10% overhead to the kernel even when tracing is

When an architecture supports DYNAMIC_FTRACE there really is no reason to
use the static tracing. I have kept non DYNAMIC_FTRACE available in x86 so
that the generic code for non DYNAMIC_FTRACE can be tested. There is no
reason to support non DYNAMIC_FTRACE for both x86_64 and x86_32. As the non
DYNAMIC_FTRACE for x86_32 does not even support fentry, and we want to
remove mcount completely, there's no reason to keep non DYNAMIC_FTRACE
around for x86_32.

Signed-off-by: Steven Rostedt (VMware) <>
arch/x86/Kconfig | 11 +++++++++++
arch/x86/kernel/ftrace_32.S | 39 -------------------------------------
2 files changed, 11 insertions(+), 39 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5ad92419be19..0544041ae3a2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -31,6 +31,17 @@ config X86_64
select X86_DEV_DMA_OPS

+ def_bool y
+ depends on X86_32
+ depends on FUNCTION_TRACER
+ help
+ We keep the static function tracing (!DYNAMIC_FTRACE) around
+ in order to test the non static function tracing in the
+ generic code, as other architectures still use it. But we
+ only need to keep it around for x86_64. No need to keep it
+ for x86_32. For x86_32, force DYNAMIC_FTRACE.
# Arch settings
diff --git a/arch/x86/kernel/ftrace_32.S b/arch/x86/kernel/ftrace_32.S
index 4c8440de3355..459e6b4a19bc 100644
--- a/arch/x86/kernel/ftrace_32.S
+++ b/arch/x86/kernel/ftrace_32.S
@@ -18,8 +18,6 @@ EXPORT_SYMBOL(__fentry__)

/* mcount uses a frame pointer even if CONFIG_FRAME_POINTER is not set */
#if !defined(CC_USING_FENTRY) || defined(CONFIG_FRAME_POINTER)
@@ -170,43 +168,6 @@ GLOBAL(ftrace_regs_call)
lea 3*4(%esp), %esp /* Skip orig_ax, ip and cs */

jmp .Lftrace_ret
- cmpl $__PAGE_OFFSET, %esp
- jb ftrace_stub /* Paging not enabled yet? */
- cmpl $ftrace_stub, ftrace_trace_function
- jnz .Ltrace
- cmpl $ftrace_stub, ftrace_graph_return
- jnz ftrace_graph_caller
- cmpl $ftrace_graph_entry_stub, ftrace_graph_entry
- jnz ftrace_graph_caller
-.globl ftrace_stub
- ret
- /* taken from glibc */
- pushl %eax
- pushl %ecx
- pushl %edx
- movl 0xc(%esp), %eax
- movl 0x4(%ebp), %edx
- subl $MCOUNT_INSN_SIZE, %eax
- movl ftrace_trace_function, %ecx
- popl %edx
- popl %ecx
- popl %eax
- jmp ftrace_stub


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