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

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Message-ID: <877djr5jc3.fsf@oldenburg.str.redhat.com>
Date:   Sat, 22 May 2021 09:16:44 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Len Brown <lenb@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen via Libc-alpha <libc-alpha@...rceware.org>,
        Rich Felker <dalias@...c.org>,
        Linux API <linux-api@...r.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kyle Huey <me@...ehuey.com>, Borislav Petkov <bp@...en8.de>,
        Keno Fischer <keno@...iacomputing.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Willy Tarreau <w@....eu>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related
 features

* 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