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Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related
 features
To:     Florian Weimer <fweimer@redhat.com>,
        Dave Hansen via Libc-alpha <libc-alpha@sourceware.org>
Cc:     Len Brown <lenb@kernel.org>, 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>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <939ec057-3851-d8fb-7b45-993fa07c4cb5@intel.com>
Date:   Fri, 21 May 2021 09:14:12 -0700
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On 5/21/21 7:44 AM, Florian Weimer wrote:
> * 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?

The thing we're trying to avoid is a #NM exception from XFD (the new
first-use detection feature) that occurs on the first use of AMX.  XCR0
will have XCR0[AMX]=1, even if XFD is "armed" and ready to generate the #NM.