Re: x86 CPU features detection for applications (and AMX)

Dave Hansen
2021-07-08T14:19:25Z

On 7/7/21 11:05 PM, Florian Weimer wrote:
>> This looks basically like someone dumped a bunch of CPUID bit values and
>> exposed them to applications without considering whether applications
>> would ever need them. For instance, why would an app ever care about:
>>
>> PKS – Protection keys for supervisor-mode pages.
>>
>> And how could glibc ever give applications accurate information about
>> whether PKS "is supported by the operating system"? It just plain
>> doesn't know, or at least only knows from a really weak ABI like
>> /proc/cpuinfo.
> glibc is expected to mask these bits for CPU_FEATURE_USABLE because they
> have unknown semantics (to glibc).

OK, so if I call CPU_FEATURE_USABLE(PKS) on a system *WITH* PKS
supported in the operating system, I'll get false from an interface that
claims to be:

> This macro returns a nonzero value (true) if the processor has the
> feature name and the feature is supported by the operating system.

The interface just seems buggy by *design*.