V6 [PATCH] x86: Add <sys/platform/x86.h>

[H.J. Lu]

On 10/24/18, Florian Weimer <fweimer@redhat.com> wrote:
> * H. J. Lu:
>
>> On 10/24/18, Florian Weimer <fweimer@redhat.com> wrote:
>>> * H. J. Lu:
>>>
>>>> On 10/24/18, Florian Weimer <fweimer@redhat.com> wrote:
>>>>> * H. J. Lu:
>>>>>
>>>>>> +@defmac HAS_CPU_FEATURE(name)
>>>>>> +
>>>>>> +Evaluate to true if the CPU feature @code{name} is supported as
>>>>>> indicated
>>>>>> +by @code{CPUID} instruction.  The available features are:
>>>>>
>>>>> I'm still puzzled why we expose this function.  It will only confuse
>>>>> programmers.
>>>>
>>>> A use case:  AVX512 bits can be used to tell Xeon Phi from Skylake
>>>> sever even if AVX512 instructions aren't used/needed at all.
>>>
>>> Do you mean checking fore AVX512ER?  Would you consider this good
>>> practice?
>>>
>>> It is actually future-proof?  What if Skylake successors implement
>>> AVX512ER?  Do you really want applications to select the Xeon Phi
>>> workarounds in this case?
>>>
>>> I still think if you care about Xeon Phi, you should use <cpuid.h> from
>>> GCC and make the checks as explicit as possible.  The <cpuid.h>
>>> facilities should not be too difficult to use for that purpose.
>>
>> Xeon Phi is an example. There are other cases where one may want to
>> know if a feature exists in CPU, not necessarily wants/needs to use the
>> feature.  The main purpose of <sys/platform/x86.h> is to make it easier
>> to check if a feature is usable or exists.
>
> I think this interface should be targeted at discovering features, not
> CPU models, to avoid confusion.
>
> If you need to figure out the model, you really need to look at CPUID
> results.  Everything else will be brittle, due to evolving CPU lines and
> virtualization.

Here is the patch with CPU_FEATURE_USABLE only.  OK for master
branch?

-- 
H.J.
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