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SubjectRe: [PATCH v2 6/6] kernel: tracepoints: add support for relative references
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)

> /*
> * 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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