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

Dave Hansen
2021-06-23T15:33:13Z

On 6/23/21 8:04 AM, Florian Weimer wrote:
> https://www.gnu.org/software/libc/manual/html_node/X86.html
...
> Previously kernel developers have expressed dismay that we didn't
> coordinate the interface with them. This is why I want raise this now.

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.

It also doesn't seem to tell applications what they want which is, "can
I, the application, *use* this feature?"

> PS: Is it true that there is no public mailing list for Linux
> discussions specific to x86?

Yes. I've asked recently for something x86-related, but folks were to
concerned what I was asking for was too specific, which was more of a
brainstorming place to put x86-specific RFC's.