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SubjectRe: [PATCH v6 5/7] arm64: ftrace: Add dynamic ftrace support
On 03/14/2014 03:10 AM, Will Deacon wrote:
> On Thu, Mar 13, 2014 at 10:13:48AM +0000, AKASHI Takahiro wrote:
>> 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 6b3fef6..6954959 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -33,6 +33,7 @@ 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_FTRACE_MCOUNT_RECORD
>> select HAVE_FUNCTION_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;
>> +}
>
> You could just as easily implement this as a dummy macro, but I guess it
> doesn't matter either way.

FYI, all archs define this as an inline function.
Leave it as it is.

>> #endif /* __ASSEMBLY__ */
>>
>> #endif /* __ASM_FTRACE_H */
>> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
>> index 0ac31c8..c0fbe10 100644
>> --- a/arch/arm64/kernel/entry-ftrace.S
>> +++ b/arch/arm64/kernel/entry-ftrace.S
>> @@ -86,6 +86,7 @@
>> add \reg, \reg, #8
>> .endm
>>
>> +#ifndef CONFIG_DYNAMIC_FTRACE
>> /*
>> * void _mcount(unsigned long return_address)
>> * @return_address: return address to instrumented function
>> @@ -134,6 +135,48 @@ 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)
>
> Judging by your comment then, this should never be called. Is that right? If
> so, we could add a BUG-equivalent so we know if we missed an mcount during
> patching.

Steven explained this.

>> +/*
>> + * 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;
>
> u32 is a bit clearer for instructions.

Fix it.

>> + /*
>> + * 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;
>
> I think you're better off propagating the errors here, rather than
> overriding them with EFAULT/EINVAL/EPERM.

Steven explained this.

>> +
>> + 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);
>
> You could rewrite this as:
>
> if (enable)
> return ftrace_modify_code(pc, nop, branch, true);
> else
> return ftrace_modify_code(pc, branch, nop, true);
>
> which I find easier to read.

Yep, fix it.

-Takahiro AKASHI

> Will
>


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