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

On Wed, Jun 23, 2021 at 05:04:27PM +0200, Florian Weimer wrote:

Copying in Catalin & Will.

We have an interface in glibc to query CPU features:

  X86-specific Facilities
  <https://www.gnu.org/software/libc/manual/html_node/X86.html>

CPU_FEATURE_USABLE all preconditions for a feature are met,
HAS_CPU_FEATURE means it's in silicon but possibly dormant.
CPU_FEATURE_USABLE is supposed to look at XCR0, AT_HWCAP2 etc. before
enabling the relevant bit (so it cannot pass through any unknown bits).

...

When we designed this glibc interface, we assumed that bits would be
static during the life-time of the process, initialized at process
start.  That follows the model of previous x86 CPU feature enablement.

...

This still wouldn't cover the enable/disable side, but at least it would
work for CPU features which are modal and come and go.  The fact that we
tell GCC to cache the returned pointer from that internal function, but
not that the data is immutable works to our advantage here.

On the other hand, maybe there is a way to give users a better
interface.  Obviously we want to avoid a syscall for a simple CPU
feature check.  And we also need something to enable/disable CPU
features.

This enabling and disabling of CPU features sounds like something that
might also become relevant for arm64, for example I can see a use case
for having something that allows some of the more expensive features
to be masked from some userspace processes for resource management
purposes.  This sounds like a bit of a different use case to x86 AIUI
but I think there's overlap in the actual operations that would be
needed.