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

On May 22, 2021, at 12:17 AM, Florian Weimer [off-list ref] wrote:

* Len Brown:

quoted

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.

Making this work well would be very nasty. The memory *must* be
available at context switch out time, which means it would need to be
pinned at context switch in time, which is not great.

But also Intel, in its infinite wisdom, decided to mix “supervisor”
states in which the state that user space is permitted to directly
access. Putting the supervisor state on the stack would be
problematic.