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Date: Sat, 2 Nov 2019 17:49:00 +0530
From: Amit Daniel Kachhap <amit.kachhap@....com>
To: Mark Rutland <mark.rutland@....com>,
linux-arm-kernel@...ts.infradead.org,
Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, catalin.marinas@....com,
deller@....de, duwe@...e.de, James.Bottomley@...senPartnership.com,
james.morse@....com, jeyu@...nel.org, jpoimboe@...hat.com,
jthierry@...hat.com, linux-parisc@...r.kernel.org,
mingo@...hat.com, peterz@...radead.org, svens@...ckframe.org,
takahiro.akashi@...aro.org, will@...nel.org
Subject: Re: [PATCHv2 1/8] ftrace: add ftrace_init_nop()
Hi Mark,
On 10/29/19 10:28 PM, Mark Rutland wrote:
> Architectures may need to perform special initialization of ftrace
> callsites, and today they do so by special-casing ftrace_make_nop() when
> the expected branch address is MCOUNT_ADDR. In some cases (e.g. for
> patchable-function-entry), we don't have an mcount-like symbol and don't
s/an mcount/a mcount.
> want a synthetic MCOUNT_ADDR, but we may need to perform some
> initialization of callsites.
>
> To make it possible to separate initialization from runtime
> modification, and to handle cases without an mcount-like symbol, this
Same as above.
> patch adds an optional ftrace_init_nop() function that architectures can
> implement, which does not pass a branch address.
>
> Where an architecture does not provide ftrace_init_nop(), we will fall
> back to the existing behaviour of calling ftrace_make_nop() with
> MCOUNT_ADDR.
>
> At the same time, ftrace_code_disable() is renamed to
> ftrace_nop_initialize() to make it clearer that it is intended to
> intialize a callsite into a disabled state, and is not for disabling a
> callsite that has been runtime enabled. The kerneldoc description of rec
> arguments is updated to cover non-mcount callsites.
>
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Torsten Duwe <duwe@...e.de>
> ---
> include/linux/ftrace.h | 35 ++++++++++++++++++++++++++++++++---
> kernel/trace/ftrace.c | 6 +++---
> 2 files changed, 35 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 8a8cb3c401b2..9867d90d635e 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -499,7 +499,7 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
> /**
> * ftrace_make_nop - convert code into nop
> * @mod: module structure if called by module load initialization
> - * @rec: the mcount call site record
> + * @rec: the call site record (e.g. mcount/fentry)
> * @addr: the address that the call site should be calling
> *
> * This is a very sensitive operation and great care needs
> @@ -520,9 +520,38 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
> extern int ftrace_make_nop(struct module *mod,
> struct dyn_ftrace *rec, unsigned long addr);
>
> +
> +/**
> + * ftrace_init_nop - initialize a nop call site
> + * @mod: module structure if called by module load initialization
> + * @rec: the call site record (e.g. mcount/fentry)
> + *
> + * This is a very sensitive operation and great care needs
> + * to be taken by the arch. The operation should carefully
> + * read the location, check to see if what is read is indeed
> + * what we expect it to be, and then on success of the compare,
> + * it should write to the location.
> + *
> + * The code segment at @rec->ip should contain the contents created by
> + * the compiler
Nit: Will it be better to write it as "@rec->ip should store the
adjusted ftrace entry address of the call site" or something like that.
> + *
> + * Return must be:
> + * 0 on success
> + * -EFAULT on error reading the location
> + * -EINVAL on a failed compare of the contents
> + * -EPERM on error writing to the location
> + * Any other value will be considered a failure.
> + */
> +#ifndef ftrace_init_nop
> +static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
> +{
> + return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
> +}
> +#endif
> +
Now that ftrace_init_nop is also an arch implemented function so it may
be added in Documentation/trace/ftrace-design.rst along with
ftrace_make_nop.
In general also, adding some description about patchable-function-entry
in kernel Documentation will be useful.
Thanks,
Amit Daniel
> /**
> * ftrace_make_call - convert a nop call site into a call to addr
> - * @rec: the mcount call site record
> + * @rec: the call site record (e.g. mcount/fentry)
> * @addr: the address that the call site should call
> *
> * This is a very sensitive operation and great care needs
> @@ -545,7 +574,7 @@ extern int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr);
> #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> /**
> * ftrace_modify_call - convert from one addr to another (no nop)
> - * @rec: the mcount call site record
> + * @rec: the call site record (e.g. mcount/fentry)
> * @old_addr: the address expected to be currently called to
> * @addr: the address to change to
> *
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index f296d89be757..5259d4dea675 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -2494,14 +2494,14 @@ struct dyn_ftrace *ftrace_rec_iter_record(struct ftrace_rec_iter *iter)
> }
>
> static int
> -ftrace_code_disable(struct module *mod, struct dyn_ftrace *rec)
> +ftrace_nop_initialize(struct module *mod, struct dyn_ftrace *rec)
> {
> int ret;
>
> if (unlikely(ftrace_disabled))
> return 0;
>
> - ret = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
> + ret = ftrace_init_nop(mod, rec);
> if (ret) {
> ftrace_bug_type = FTRACE_BUG_INIT;
> ftrace_bug(ret, rec);
> @@ -2943,7 +2943,7 @@ static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs)
> * to the NOP instructions.
> */
> if (!__is_defined(CC_USING_NOP_MCOUNT) &&
> - !ftrace_code_disable(mod, p))
> + !ftrace_nop_initialize(mod, p))
> break;
>
> update_cnt++;
>
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