Re: [PATCH] Add ifunc memcpy and memmove for aarch64

[Andrew Pinski <pinskia at gmail dot com>]

On Thu, Feb 9, 2017 at 2:50 AM, Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> On 09/02/17 00:02, Steve Ellcey wrote:
>> +static inline void __attribute__ ((unused))
>> +dl_platform_init (void)
>> +{
>> +  if (GLRO(dl_platform) != NULL && *GLRO(dl_platform) == '\0')
>> +    /* Avoid an empty string which would disturb us.  */
>> +    GLRO(dl_platform) = NULL;
>> +
>> +#ifdef SHARED
>> +  /* init_cpu_features has been called early from __libc_start_main in
>> +     static executable.  */
>> +  init_cpu_features (&GLRO(dl_aarch64_cpu_features));
>> +#endif
>> +}
> ...
>> +static inline void
>> +init_cpu_features (struct cpu_features *cpu_features)
>> +{
>> +  if (GLRO(dl_hwcap) & HWCAP_CPUID)
>> +    {
>> +      register uint64_t id = 0;
>> +      asm volatile ("mrs %0, midr_el1" : "=r"(id));
>> +      cpu_features->midr_el1 = id;
>
> this is a trap into the kernel at every process startup
>
> since this is called very early (dynamic linking case
> above, static linking case below) i wonder if there
> could be a way for the user to request midr_el1==0
> unconditionally (avoiding the overhead and making
> sure the most generic implementation is used)

Well the easy way to do this would be use LD_HWCAP_MASK and mask off
the HWCAP_CPUID bit.

>
> is there something like that on other targets?

Some targets like PowerPC use the hwcap to say which processor they
are being run on so you use the same method as above.

Thanks,
Andrew

>
>> +    }
>> +  else
>> +    {
>> +      cpu_features->midr_el1 = 0;
>> +    }
>> +}
> ...
>> +#ifdef SHARED
>> +# include <csu/libc-start.c>
>> +# else
>> +/* The main work is done in the generic function.  */
>> +# define LIBC_START_DISABLE_INLINE
>> +# define LIBC_START_MAIN generic_start_main
>> +# include <csu/libc-start.c>
>> +# include <cpu-features.c>
>> +
>> +extern struct cpu_features _dl_aarch64_cpu_features;
>> +
>> +int
>> +__libc_start_main (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
>> +                int argc, char **argv,
>> +                __typeof (main) init,
>> +                void (*fini) (void),
>> +                void (*rtld_fini) (void), void *stack_end)
>> +{
>> +  init_cpu_features (&_dl_aarch64_cpu_features);
>> +  return generic_start_main (main, argc, argv, init, fini, rtld_fini,
>> +                          stack_end);
>> +}
>> +#endif
>>
>