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From:   Florian Weimer <fweimer@redhat.com>
To:     Dave Hansen via Libc-alpha <libc-alpha@sourceware.org>
Cc:     Len Brown <lenb@kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        Rich Felker <dalias@libc.org>,
        Linux API <linux-api@vger.kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Kyle Huey <me@kylehuey.com>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.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> <20210415052938.GA2325@1wt.eu>
        <20210415054713.GB6318@zn.tnic>
        <CAJvTdKnjzAMh3N_c7KP3kA=e0LgYHgCANg44oJp3LcSm7dtbSQ@mail.gmail.com>
        <20210419141454.GE9093@zn.tnic>
        <CAJvTdK=p8mgO3xw9sRxu0c7NTNTG109M442b3UZh8TqLLfkC1Q@mail.gmail.com>
        <20210419191539.GH9093@zn.tnic>
        <CAJvTdK=VnG94ECcRVoUi8HrCbVEKc8X4_JmRTkqe+vTttf0Wsg@mail.gmail.com>
        <20210419215809.GJ9093@zn.tnic>
        <CAJvTdKn6JHo02karEs0e5g+6SimS5VUcXKjCkX35WY+xkgAgxw@mail.gmail.com>
        <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>
Date:   Fri, 21 May 2021 16:44:58 +0200
Message-ID: <87pmxk87th.fsf@oldenburg.str.redhat.com>
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* Dave Hansen via Libc-alpha:

> Our system calls are *REALLY* fast.  We can even do a vsyscall for this
> if we want to get the overhead down near zero.  Userspace can also cache
> the "I did the prctl()" state in thread-local storage if it wants to
> avoid the syscall.

Why can't userspace look at XCR0 to make the decision?

And we added an interface for querying x86 CPU features to glibc 2.33
which is completely incompatible with this because it assumes that CPU
features do not change during the lifetime of a process. 8-(

Thanks,
Florian