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Subject[PATCH v4 2/7] arm64: ftrace: Add dynamic ftrace support
Date
This patch allows "dynamic ftrace" if CONFIG_DYNAMIC_FTRACE is enabled.
Here we can turn on and off tracing dynamically per-function base.

On arm64, this is done by patching single branch instruction to _mcount()
inserted by gcc -pg option. The branch is replaced to NOP initially at
kernel start up, and later on, NOP to branch to ftrace_caller() when
enabled or branch to NOP when disabled.
Please note that ftrace_caller() is a counterpart of _mcount() in case of
'static' ftrace.

More details on architecture specific requirements are described in
Documentation/trace/ftrace-design.txt.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
---
arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/ftrace.h | 15 +++++
arch/arm64/kernel/entry-ftrace.S | 43 +++++++++++++++
arch/arm64/kernel/ftrace.c | 113 ++++++++++++++++++++++++++++++++++++++
4 files changed, 172 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 5783641..b176bc0 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -32,6 +32,7 @@ config ARM64
select HAVE_DMA_API_DEBUG
select HAVE_DMA_ATTRS
select HAVE_DMA_CONTIGUOUS
+ select HAVE_DYNAMIC_FTRACE
select HAVE_EFFICIENT_UNALIGNED_ACCESS
select HAVE_FUNCTION_TRACER
select HAVE_FUNCTION_GRAPH_TRACER
diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 58ea595..ed5c448 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -18,6 +18,21 @@

#ifndef __ASSEMBLY__
extern void _mcount(unsigned long);
+
+struct dyn_arch_ftrace {
+ /* No extra data needed for arm64 */
+};
+
+extern unsigned long ftrace_graph_call;
+
+static inline unsigned long ftrace_call_adjust(unsigned long addr)
+{
+ /*
+ * addr is the address of the mcount call instruction.
+ * recordmcount does the necessary offset calculation.
+ */
+ return addr;
+}
#endif /* __ASSEMBLY__ */

#endif /* __ASM_FTRACE_H */
diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
index 2e8162e..b948878 100644
--- a/arch/arm64/kernel/entry-ftrace.S
+++ b/arch/arm64/kernel/entry-ftrace.S
@@ -84,6 +84,7 @@
add \reg, \reg, #8
.endm

+#ifndef CONFIG_DYNAMIC_FTRACE
/*
* void _mcount(unsigned long return_address)
* @return_address: return address to instrumented function
@@ -132,6 +133,48 @@ skip_ftrace_call:
#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
ENDPROC(_mcount)

+#else /* CONFIG_DYNAMIC_FTRACE */
+/*
+ * _mcount() is used to build the kernel with -pg option, but all the branch
+ * instructions to _mcount() are replaced to NOP initially at kernel start up,
+ * and later on, NOP to branch to ftrace_caller() when enabled or branch to
+ * NOP when disabled per-function base.
+ */
+ENTRY(_mcount)
+ ret
+ENDPROC(_mcount)
+
+/*
+ * void ftrace_caller(unsigned long return_address)
+ * @return_address: return address to instrumented function
+ *
+ * This function is a counterpart of _mcount() in 'static' ftrace, and
+ * makes calls to:
+ * - tracer function to probe instrumented function's entry,
+ * - ftrace_graph_caller to set up an exit hook
+ */
+ENTRY(ftrace_caller)
+ mcount_enter
+
+ mcount_get_pc0 x0 // function's pc
+ mcount_get_lr x1 // function's lr
+
+ .global ftrace_call
+ftrace_call: // tracer(pc, lr);
+ nop // This will be replaced with "bl xxx"
+ // where xxx can be any kind of tracer.
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+ .global ftrace_graph_call
+ftrace_graph_call: // ftrace_graph_caller();
+ nop // If enabled, this will be replaced
+ // "b ftrace_graph_caller"
+#endif
+
+ mcount_exit
+ENDPROC(ftrace_caller)
+#endif /* CONFIG_DYNAMIC_FTRACE */
+
ENTRY(ftrace_stub)
ret
ENDPROC(ftrace_stub)
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index a559ab8..8c26476 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -17,6 +17,89 @@
#include <asm/ftrace.h>
#include <asm/insn.h>

+#ifdef CONFIG_DYNAMIC_FTRACE
+/*
+ * Replace a single instruction, which may be a branch or NOP.
+ * If @validate == true, a replaced instruction is checked against 'old'.
+ */
+static int ftrace_modify_code(unsigned long pc, unsigned int old,
+ unsigned int new, bool validate)
+{
+ unsigned int replaced;
+
+ /*
+ * Note:
+ * Due to modules and __init, code can disappear and change,
+ * we need to protect against faulting as well as code changing.
+ * We do this by aarch64_insn_*() which use the probe_kernel_*().
+ *
+ * No lock is held here because all the modifications are run
+ * through stop_machine().
+ */
+ if (validate) {
+ if (aarch64_insn_read((void *)pc, &replaced))
+ return -EFAULT;
+
+ if (replaced != old)
+ return -EINVAL;
+ }
+ if (aarch64_insn_patch_text_nosync((void *)pc, new))
+ return -EPERM;
+
+ return 0;
+}
+
+/*
+ * Replace tracer function in ftrace_caller()
+ */
+int ftrace_update_ftrace_func(ftrace_func_t func)
+{
+ unsigned long pc;
+ unsigned int new;
+
+ pc = (unsigned long)&ftrace_call;
+ new = aarch64_insn_gen_branch_imm(pc, (unsigned long)func, true);
+
+ return ftrace_modify_code(pc, 0, new, false);
+}
+
+/*
+ * Turn on the call to ftrace_caller() in instrumented function
+ */
+int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
+{
+ unsigned long pc = rec->ip;
+ unsigned int old, new;
+
+ old = aarch64_insn_gen_nop();
+ new = aarch64_insn_gen_branch_imm(pc, addr, true);
+
+ return ftrace_modify_code(pc, old, new, true);
+}
+
+/*
+ * Turn off the call to ftrace_caller() in instrumented function
+ */
+int ftrace_make_nop(struct module *mod,
+ struct dyn_ftrace *rec, unsigned long addr)
+{
+ unsigned long pc = rec->ip;
+ unsigned int old, new;
+
+ old = aarch64_insn_gen_branch_imm(pc, addr, true);
+ new = aarch64_insn_gen_nop();
+
+ return ftrace_modify_code(pc, old, new, true);
+}
+
+int __init ftrace_dyn_arch_init(void *data)
+{
+ *(unsigned long *)data = 0;
+
+ return 0;
+}
+#endif /* CONFIG_DYNAMIC_FTRACE */
+
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
/*
* function_graph tracer expects ftrace_return_to_handler() to be called
@@ -61,4 +144,34 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
return;
}
}
+
+#ifdef CONFIG_DYNAMIC_FTRACE
+/*
+ * Turn on/off the call to ftrace_graph_caller() in ftrace_caller()
+ * depending on @enable.
+ */
+static int ftrace_modify_graph_caller(bool enable)
+{
+ unsigned long pc = (unsigned long)&ftrace_graph_call;
+ unsigned int branch, nop, old, new;
+
+ branch = aarch64_insn_gen_branch_imm(pc,
+ (unsigned long)ftrace_graph_caller, false);
+ nop = aarch64_insn_gen_nop();
+ old = enable ? nop : branch;
+ new = enable ? branch : nop;
+
+ return ftrace_modify_code(pc, old, new, true);
+}
+
+int ftrace_enable_ftrace_graph_caller(void)
+{
+ return ftrace_modify_graph_caller(true);
+}
+
+int ftrace_disable_ftrace_graph_caller(void)
+{
+ return ftrace_modify_graph_caller(false);
+}
+#endif /* CONFIG_DYNAMIC_FTRACE */
#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
--
1.7.9.5


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