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SubjectRe: [PATCH v2 6/6] kernel: tracepoints: add support for relative references
On Fri, 18 Aug 2017 12:44:17 +0100
Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:

> On 18 August 2017 at 12:26, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > To avoid the need for relocating absolute references to tracepoint
> > structures at boot time when running relocatable kernels (which may
> > take a disproportionate amount of space), add the option to emit
> > these tables as relative references instead.
> >
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > ---
> > include/linux/tracepoint.h | 42 ++++++++++++++++++--
> > kernel/tracepoint.c | 7 +---
> > 2 files changed, 40 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index a26ffbe09e71..68701821933a 100644
> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -228,6 +228,42 @@ extern void syscall_unregfunc(void);
> > return static_key_false(&__tracepoint_##name.key); \
> > }
> >
> > +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> > +#define __TRACEPOINT_ENTRY(name) \
> > + asm(" .section \"__tracepoints_ptrs\", \"a\"\n" \
> > + " .balign 4\n" \
> > + " .long " VMLINUX_SYMBOL_STR(__tracepoint_##name) " - .\n"\
> > + " .previous\n")
> > +
> > +struct tracepoint_entry_t {
> > + int tp_offset;
> > +};
> > +
> > +static inline
> > +struct tracepoint *tracepoint_from_entry(const struct tracepoint_entry_t *ent)
> > +{
> > + return (struct tracepoint *)((unsigned long)ent + ent->tp_offset);
> > +}
> > +#else
> > +#define __TRACEPOINT_ENTRY(name) \
> > + static struct tracepoint * const __tracepoint_ptr_##name __used \
> > + __attribute__((section("__tracepoints_ptrs"))) = \
> > + &__tracepoint_##name
> > +
> > +struct tracepoint_entry_t {
> > + struct tracepoint *tp;
> > +};
> > +
> > +static inline
> > +struct tracepoint *tracepoint_from_entry(const struct tracepoint_entry_t *ent)
> > +{
> > + return ent->tp;
> > +}
> > +#endif
> > +
> > +extern struct tracepoint_entry_t const __start___tracepoints_ptrs[];
> > +extern struct tracepoint_entry_t const __stop___tracepoints_ptrs[];
> > +
>
> It appears the stuff above needs to be move inside the double-include
> guard (which oddly enough does not cover the entire file)

Why was this moved to the header file? To fulfill some checkpatch
warning?

-- Steve

>
> > /*
> > * We have no guarantee that gcc and the linker won't up-align the tracepoint
> > * structures, so we create an array of pointers that will be used for iteration
> > @@ -237,11 +273,9 @@ extern void syscall_unregfunc(void);
> > static const char __tpstrtab_##name[] \
> > __attribute__((section("__tracepoints_strings"))) = #name; \
> > struct tracepoint __tracepoint_##name \
> > - __attribute__((section("__tracepoints"))) = \
> > + __attribute__((section("__tracepoints"))) __used = \
> > { __tpstrtab_##name, STATIC_KEY_INIT_FALSE, reg, unreg, NULL };\
> > - static struct tracepoint * const __tracepoint_ptr_##name __used \
> > - __attribute__((section("__tracepoints_ptrs"))) = \
> > - &__tracepoint_##name;
> > + __TRACEPOINT_ENTRY(name);
> >
> > #define DEFINE_TRACE(name) \
> > DEFINE_TRACE_FN(name, NULL, NULL);
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index 685c50ae6300..21bc41454fd6 100644
> > --- a/kernel/tracepoint.c
> > +++ b/kernel/tracepoint.c
> > @@ -28,9 +28,6 @@
> > #include <linux/sched/task.h>
> > #include <linux/static_key.h>
> >
> > -extern struct tracepoint * const __start___tracepoints_ptrs[];
> > -extern struct tracepoint * const __stop___tracepoints_ptrs[];
> > -
> > /* Set to 1 to enable tracepoint debug output */
> > static const int tracepoint_debug;
> >
> > @@ -508,12 +505,12 @@ static void for_each_tracepoint_range(struct tracepoint * const *begin,
> > void (*fct)(struct tracepoint *tp, void *priv),
> > void *priv)
> > {
> > - struct tracepoint * const *iter;
> > + struct tracepoint_entry_t const *iter;
> >
> > if (!begin)
> > return;
> > for (iter = begin; iter < end; iter++)
> > - fct(*iter, priv);
> > + fct(tracepoint_from_entry(iter), priv);
> > }
> >
> > /**
> > --
> > 2.11.0
> >

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