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

[Florian Weimer]

* Len Brown:

> A. per-task.  If we do it this way, then we will likely wind up
> mandating a GET at the start of every routine in every library that
> touches AMX, and potentially also a PUT.  This is because the library
> has no idea what thread called it.  The plus is that this will address
> the "used once and sits on a buffer for the rest of the process
> lifetime' scenario.  The minus is that high performance users will be
> executing thousands of unnecessary system calls that have zero value.

We could revive the KTLS proposal (userspace donates memory for use by
the kernel & vDSO), and the thread could reserve (on-stack) buffer space
for kernel use for the duration of the AMX computation.  There would be
a pointer to that space in the KTLS area, set upon entry of the AMX
region, and cleared upon exit.  It's not extremely cheap (unbounded
alloca has a stack probing loop nowadays).  But no system call is
required.

Thanks,
Florian