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From:   Len Brown <lenb@kernel.org>
Date:   Wed, 31 Mar 2021 18:11:31 -0400
Message-ID: <CAJvTdKkYPK2iqUPGMQZ63Qdgfy_JR5fpjQevcwJZ0Y+PHM80Cw@mail.gmail.com>
Subject: Re: Candidate Linux ABI for Intel AMX and hypothetical new related features
To:     robert@ocallahan.org
Cc:     Andy Lutomirski <luto@amacapital.net>,
        David Laight <David.Laight@aculab.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        libc-alpha <libc-alpha@sourceware.org>,
        Florian Weimer <fweimer@redhat.com>,
        Rich Felker <dalias@libc.org>, Kyle Huey <me@kylehuey.com>,
        Keno Fischer <keno@juliacomputing.com>,
        Linux API <linux-api@vger.kernel.org>
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On Wed, Mar 31, 2021 at 5:42 PM Robert O'Callahan <robert@ocallahan.org> wrote:
>
> For the record, the benefits of dynamic XCR0 for rr recording
> portability still apply. I guess it'd be useful for CRIU too. We would
> also benefit from anything that incentivizes increased support for
> CPUID faulting.

As previously mentioned, today we don't have an architectural way to
trap a user into the kernel on CPUID,
even though we can do this for a VMM.

But spoofing CPUID isn't a solution to all problems.
The feature really needs to be OFF to prevent users from using it,
even if the supported mechanisms of discovering that feature say "NOT PRESENT".

Today there are plenty of users who will opportunistically try everything
in the cloud and choose the machine that allows them to do something
that other machines will not -- even if it is not officially supported.

If something is not enumerated, it really needs to also be turned off.

cheers,
--Len Brown, Intel Open Source Technology Center