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From:   Florian Weimer <fweimer@redhat.com>
To:     Len Brown <lenb@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Dave Hansen via Libc-alpha <libc-alpha@sourceware.org>,
        Rich Felker <dalias@libc.org>,
        Linux API <linux-api@vger.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kyle Huey <me@kylehuey.com>, Borislav Petkov <bp@alien8.de>,
        Keno Fischer <keno@juliacomputing.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Willy Tarreau <w@1wt.eu>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related
 features
References: <20210415044258.GA6318@zn.tnic> <20210419191539.GH9093@zn.tnic>
        <CAJvTdK=VnG94ECcRVoUi8HrCbVEKc8X4_JmRTkqe+vTttf0Wsg@mail.gmail.com>
        <20210419215809.GJ9093@zn.tnic>
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        <YIMmwhEr46VPAZa4@zn.tnic>
        <CAJvTdKnhXnynybS4eNEF_EtF26auyb-mhKLNd1D9_zvCrchZsw@mail.gmail.com>
        <874kf11yoz.ffs@nanos.tec.linutronix.de>
        <CAJvTdKkYp+zP_9tna6YsrOz2_nmEUDLJaL_i-SNog0m2T9wZ=Q@mail.gmail.com>
        <87k0ntazyn.ffs@nanos.tec.linutronix.de>
        <37833625-3e6b-5d93-cc4d-26164d06a0c6@intel.com>
        <CAJvTdKmqzO4P9k3jqRA=dR+B7yV72hZCiyC8HGQxDKZBnXgzZQ@mail.gmail.com>
        <9c8138eb-3956-e897-ed4e-426bf6663c11@intel.com>
        <87pmxk87th.fsf@oldenburg.str.redhat.com>
        <939ec057-3851-d8fb-7b45-993fa07c4cb5@intel.com>
        <87r1i06ow2.fsf@oldenburg.str.redhat.com>
        <263a58a9-26d5-4e55-b3e1-3718baf1b81d@www.fastmail.com>
        <87k0nraonu.ffs@nanos.tec.linutronix.de>
        <CAJvTdK=A64DQXjYkZgPebWb-V_p_HAM+jTZRLTyi1qrP9kucMg@mail.gmail.com>
        <878s47aeni.ffs@nanos.tec.linutronix.de>
        <CAJvTdK=Ws1QvBvdx50OSmAi0vBX49KZZHUoiMFbhUUEPmjGmBw@mail.gmail.com>
Date:   Sat, 22 May 2021 09:16:44 +0200
In-Reply-To: <CAJvTdK=Ws1QvBvdx50OSmAi0vBX49KZZHUoiMFbhUUEPmjGmBw@mail.gmail.com>
        (Len Brown's message of "Fri, 21 May 2021 19:31:36 -0400")
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* 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