Messages in this thread | | | Subject | Re: [PATCHv2 1/8] ftrace: add ftrace_init_nop() | From | Amit Kachhap <> | Date | Tue, 5 Nov 2019 12:17:26 +0530 |
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Hi Mark,
On 11/4/19 7:06 PM, Mark Rutland wrote: > On Sat, Nov 02, 2019 at 05:49:00PM +0530, Amit Daniel Kachhap wrote: >> Hi Mark, > > Hi Amit, > > Thanks for the review! > >> 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. > > Using 'an' in both of these cases is correct, even though 'mcount' > starts with a consonant, since it's pronounced as-if it were 'emcount'. > I will leave this as-is. Ok sure. It makes sense. > >>> 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@arm.com> >>> Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> >>> Cc: Ingo Molnar <mingo@redhat.com> >>> Cc: Steven Rostedt <rostedt@goodmis.org> >>> Cc: Torsten Duwe <duwe@suse.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. > > This was the specific wording requested by Steve, and it's trying to > describe the instructions at rec->ip, rather than the value of rec->ip, > so I think it's better to leave this as-is. ok Its fine this way too. Actually from the comment, I could not understand which one of the compiler contents this points to as in this case there are 2 nops. > >>> + * >>> + * 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. > > I agree that would be a good thing, but as Steve suggests I will leave > this for subsequent rework, as I think that also implies more > rework/renaming in the core code (e.g. to more cleanly separate the > notion of a callsite from mcount specifically). ok.
Thanks, Amit > > Steve, if you'd like help with that (or review), I'd be happy to help. > > Thanks, > Mark. >
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