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

On 7/7/21 11:05 PM, Florian Weimer wrote:

quoted

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*.