Hello.
I have just installed a new Linux image with CONFIG_VSX=y, however nettle is
still triggering illegal instruction there.
2022-10-20 2:14 GMT+08:00, Niels Möller <ni...@lysator.liu.se>:
> 王昊然 <msl0000023...@gmail.com> writes:
>
>>> int main() {
>>> unsigned long int hwcap = getauxval(AT_HWCAP);
>>> printf("hwcap = 0x%lx\n", hwcap);
>>> return 0;
>>> }
>>> whr@debian:~/src$ gcc -Wall cpu-feature-test.c
>>> whr@debian:~/src$ ./a.out
>>> hwcap = 0xdc0065c2
>
> And the flags checked by nettle are (from
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/include/uapi/asm/cputable.h?h=v6.1-rc1)
>
>
> #define PPC_FEATURE_HAS_ALTIVEC 0x10000000
> #define PPC_FEATURE_HAS_VSX 0x00000080
>
> so both set in your case.
>
>>>
>>>> Is the program that crashes running under a vm, or is the kernel
>>>> running
>>>> on the bare metal? Each layer of vm tends to be ian opportunity to
>>>> introduce
>>>> errors in the list of available processor features.
>>>
>>> Operating systems on this machine are always running on the Power
>>> Hypervisor,
>>> which is a part of the server firmware.
>>>
>>
>> I just checked the nettle source code, it is indeed correctly checked
>> both
>> PPC_FEATURE_HAS_ALTIVEC and PPC_FEATURE_HAS_VSX; and with my HWCAP has
>> PPC_FEATURE_HAS_VSX set, I think it hits a limitation of this processor
>> feature checking logic: hardware supporting it, but the kernel didn't.
>>
>> I will try to build a new kernel with CONFIG_VSX enabled by tomorrow.
>
> My understanding is that AT_HWCAP should describe what instructions are
> available to userspace processes. If an instruction set extension isn't
> fully supported and enabled by all of hw/kernel/hypervisor, it should
> not be listed in hwcap. So the alternative might be to leave VSX
> disabled, but ensure it isn't advertised in hwcaps.
>
> Regards,
> /Niels
>
> --
> Niels Möller. PGP key CB4962D070D77D7FCB8BA36271D8F1FF368C6677.
> Internet email is subject to wholesale government surveillance.
>
