Re: [PATCH v1 2/5] perf parse-regs: Introduce functions arch__reg_{ip|sp}()

[James Clark <james.clark@arm.com>]

On 20/05/2023 03:55, Leo Yan wrote:
> Ideally, we want util/perf_regs.c to be general enough and doesn't bind
> with specific architecture.
> 
> But since util/perf_regs.c uses the macros PERF_REG_IP and PERF_REG_SP
> which are defined by architecture, thus util/perf_regs.c is dependent on
> architecture header (see util/perf_regs.h includes "<perf_regs.h>", here
> perf_regs.h is architecture specific header).
> 
> As a step to generalize util/perf_regs.c, this commit introduces weak
> functions arch__reg_ip() and arch__reg_sp() and every architecture can
> define their own functions; thus, util/perf_regs.c doesn't need to use
> PERF_REG_IP and PERF_REG_SP anymore.
> 
> This is a preparation to get rid of architecture specific header from
> util/perf_regs.h.
> 
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
[...]
>  
> -#define DWARF_MINIMAL_REGS ((1ULL << PERF_REG_IP) | (1ULL << PERF_REG_SP))
> +#define DWARF_MINIMAL_REGS ((1ULL << arch__reg_ip()) | (1ULL << arch__reg_sp()))
>  
>  const char *perf_reg_name(int id, const char *arch);
>  int perf_reg_value(u64 *valp, struct regs_dump *regs, int id);
> diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.c
> index bdccfc511b7e..f308f2ea512b 100644
> --- a/tools/perf/util/unwind-libdw.c
> +++ b/tools/perf/util/unwind-libdw.c
> @@ -252,7 +252,7 @@ int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
>  	if (!ui->dwfl)
>  		goto out;
>  
> -	err = perf_reg_value(&ip, &data->user_regs, PERF_REG_IP);
> +	err = perf_reg_value(&ip, &data->user_regs, arch__reg_ip());

Shouldn't it be more like this, because the weak symbols are a compile
time thing and it's supposed to support cross arch unwinding at runtime
(assuming something containing the arch from the file is passed down,
like we did with perf_reg_name()):

  char *arch = perf_env__arch(evsel__env(evsel));
  err = perf_reg_value(&ip, &data->user_regs, arch__reg_ip(arch));

Now I'm wondering how cross unwinding ever worked because I see
libunwind also has something hard coded too:

  #define LIBUNWIND__ARCH_REG_SP PERF_REG_SP