[PATCH x86_64][BZ #20139] Don't allow configure with not supporting AVX512 assembler w/o --disable-avx512.

[Florian Weimer]

On 06/27/2016 08:39 PM, H.J. Lu wrote:
> On Mon, Jun 27, 2016 at 11:33 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 06/27/2016 08:29 PM, H.J. Lu wrote:
>>
>>> If ld.so doesn't the first 8 save/store ZMM registers, you
>>> may not pass parameters in ZMM registers with AVX512
>>> kernel on AVX512 machine.   We wan to make sure that
>>> ld.so in x86-64 glibc saves/stores ZMM registers unless
>>> glibc is configured to disable AVX512 support.
>>
>>
>> I still do not understand what is going on.
>>
>> What can clobber the ZMM registers if glibc was compiled without AVX-512
>> capabilities?
>
> When AVX512 isn't supported, _dl_runtime_resolve_avx will be to used
> to save the first 8 vector registers, which only saves the lower 256 bits of
> vector register, for lazy binding.  When it is called on AVX512 platform,
> the upper 256 bits of ZMM registers are clobbered.

Thanks, I wasn't aware of the register sharing.

So this affects all existing glibc binaries, as long as they are 
compiled with AVX support.  So applications really need to detect this 
somehow before they can use AVX512 features.  Or binaries which use 
AVX512 unconditionally need to be marked in some way so that ld.so will 
refuse to load them.

Florian