Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

On Mon, May 17 2021 at 11:56, Florian Weimer wrote:

* Thomas Gleixner:

quoted

Having a proper interface (syscall, prctl) which user space can use to
ask for permission and allocation of the necessary buffer(s) is clearly
avoiding the downsides and provides the necessary mechanisms for proper
control and failure handling.

It's not the end of the world if something which wants to utilize this
has do issue a syscall during detection. It does not matter whether
that's a library or just the application code itself.

That's a one off operation and every involved entity can cache the
result in TLS.

I'm not sure if it's a good idea to have each AMX consumer to set up its
own TLS cache.  How expensive is checking XCR0 via XGETBV instead on the
AMX path?  Then AMX can be enabled on the thread via a system call. 

Right, did not think about that.

It also allows disabling of AMX.

That needs reference counting, but yes it's possible.

It would also need an AT_HWCAP2 feature flag telling user space that AMX
support is available after that system call (switching on AMX to check
whether AMX paths should enabled later seems potentially wasteful if the
AMX paths are never taken after all).

Either that or just have:

       prctl(PR_QUERY_XSTATE_FEATURES,....
       prctl(PR_ENABLE_XSTATE_FEATURES,....
       prctl(PR_DISABLE_XSTATE_FEATURES,....

Thanks,

        tglx