Re: [PATCH v3] AArch64: Add ifunc masking tunable

On 14/07/26 07:58, Yury Khrustalev wrote:
> On Mon, Jul 06, 2026 at 12:49:16PM +0000, Wilco Dijkstra wrote:
>> v3: keep the tst-cpu-tunable-static-pie test
>>
>> Remove the glibc.cpu.name tunable since it's unused and out of date.
>> Add support for glibc.cpu.hwcaps to adjust ifunc selection for debugging
>> and benchmarking (undocumented since this is for developers only).
> 
> The 'glibc.cpu.hwcaps' tunable is documented in the Glibc manual with
> this note:
> 
>   This tunable is specific to i386, x86-64, s390x, powerpc and
>   loongarch.
> 
> Now in this patch we are adding support for aarch64 but in a way that
> does not follow what is documented for other targets. This might result
> in confusion. Maybe we should add a comment to the manual that this
> tunable is partially supported on aarch64 but only allows to turn off
> some hwcaps and only those that don't affect security?

Agree, we should proper document the way aarch64 deviates from other
ABIs.

> 
>> Only allow disabling of features that are (a) used by ifuncs, (b) safe
>> to disable to a more generic ifunc without any security impact.
>>
>> ...
>>
>> diff --git a/sysdeps/aarch64/dl-tunables.list b/sysdeps/aarch64/dl-tunables.list
>> index a2ccba0b293a2d728ec6986f7ddd5b9793c1de17..c876f3fe0d4891be425f0c022e4adcaa11f86af4 100644
>> --- a/sysdeps/aarch64/dl-tunables.list
>> +++ b/sysdeps/aarch64/dl-tunables.list
>> @@ -18,7 +18,7 @@
>>  
>>  glibc {
>>    cpu {
>> -    name {
>> +    hwcaps {
>>        type: STRING
>>      }
> 
> OK
> 
>>      aarch64_bti {
>> diff --git a/sysdeps/unix/sysv/linux/aarch64/Makefile b/sysdeps/unix/sysv/linux/aarch64/Makefile
>> index d1fcb48aa250dffd8b7e5b41f5b290512dc797e2..baefac15b5985414a5044766048cab5e84e4636f 100644
>> --- a/sysdeps/unix/sysv/linux/aarch64/Makefile
>> +++ b/sysdeps/unix/sysv/linux/aarch64/Makefile
>> @@ -17,7 +17,7 @@ tests += \
>>  tests-static += \
>>    tst-cpu-tunable-static-pie \
>>    # tests-static
>> -tst-cpu-tunable-static-pie-TUNABLES = glibc.cpu.name=generic
>> +tst-cpu-tunable-static-pie-TUNABLES = glibc.cpu.hwcaps=-midr,-sve,-mops
>>  endif
>>  endif
> 
> OK
> 
>>  
>> diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
>> index 9a87332ac49c0272930200ece5276eb5a4a2fae4..6a13638368673870b9a9e35bbb43108612561ae9 100644
>> --- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
>> +++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
>> @@ -33,56 +33,39 @@
>>     to see when pointer have been correctly tagged.  */
>>  #define MTE_ALLOWED_TAGS (0xfffe << PR_MTE_TAG_SHIFT)
>>  
>> -static const char cpu_list_name[] = {
>> -  "kunpeng920\0"
>> -  "kunpeng950\0"
>> -  "a64fx\0"
>> -  "generic\0",
>> -};
>> -
>> -static const uint64_t cpu_list_midr[] = {
>> -  0x481FD010,
>> -  0x480FD060,
>> -  0x460F0010,
>> -  0x0,
>> -};
> 
> OK
> 
>> -
>> -static uint64_t
>> -get_midr_from_mcpu (const struct tunable_str_t *mcpu)
>> +static void
>> +TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *val)
>>  {
>> -  const char *name = cpu_list_name;
>> -  size_t offset = 0;
>> -  for (int i = 0; i < array_length (cpu_list_midr); i++)
>> +  struct cpu_features *cpu_features = &GLRO(dl_aarch64_cpu_features);
>> +  struct tunable_str_comma_state_t cs;
>> +  tunable_str_comma_init (&cs, val);
>> +
>> +  struct tunable_str_comma_t n;
>> +  while (tunable_str_comma_next (&cs, &n))
>>      {
>> -      size_t len = strlen (name);
>> -      if (tunable_strcmp (mcpu, cpu_list_name + offset, len))
>> -	return cpu_list_midr[i];
>> -      offset += len;
>> +      /* Support disabling of features to select more generic ifuncs.  */
>> +      if (!n.disable)
>> +	continue;
>> +      if (tunable_str_comma_strcmp_cte (&n, "midr"))
> 
> Should this (and similar values below) be '-midr' with the '-' used to
> make sure we only support disabling?
> 
>> +	cpu_features->midr_el1 = 0;
>> +      else if (tunable_str_comma_strcmp_cte (&n, "zva"))
>> +	cpu_features->zva_size = 0;
>> +      else if (tunable_str_comma_strcmp_cte (&n, "sve"))
>> +	cpu_features->sve = false;
>> +      else if (tunable_str_comma_strcmp_cte (&n, "sve2"))
>> +	cpu_features->sve2 = false;
>> +      else if (tunable_str_comma_strcmp_cte (&n, "mops"))
>> +	cpu_features->mops = false;
>>      }
>> -  return UINT64_MAX;
>>  }
>>  
>> ...
>>
>> @@ -108,4 +91,8 @@ init_cpu_features (struct cpu_features *cpu_features)
>>    if (GLRO (dl_hwcap) & HWCAP_GCS)
>>      /* GCS status may be updated later by binary compatibility checks.  */
>>      GL (dl_aarch64_gcs) = TUNABLE_GET (glibc, cpu, aarch64_gcs, uint64_t, 0);
>> +
>> +  /* Allow override by glibc.cpu.hwcaps tunable after setting features.  */
>> +  TUNABLE_GET (glibc, cpu, hwcaps, tunable_val_t *,
>> +	       TUNABLE_CALLBACK (set_hwcaps));
>>  }
> 
> OK
> 
> Thanks,
> Yury
>