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From: Dan Williams <dan.j.williams@intel.com>
Date: Sun, 7 Jan 2018 12:15:40 -0800
Message-ID: <CAPcyv4gm0tAbLHHyMvsbghV+SpnbaTCmJd6N8imwUnRZ9k2xUA@mail.gmail.com>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Willy Tarreau <w@1wt.eu>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch@vger.kernel.org, Andi Kleen <ak@linux.intel.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Network Development <netdev@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org

On Sun, Jan 7, 2018 at 11:47 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Sat, Jan 6, 2018 at 10:33 PM, Willy Tarreau <w@1wt.eu> wrote:
>>
>> To be fair there's overreaction on both sides. The vast majority of
>> users need to get a 100% safe system and will never notice  any
>> difference.
>
> There is no such thing as a "100% safe system". Never will be - unless
> you make sure you have no users.
>
> Also, people definitely *are* noticing the performance issues with the
> current set of patches, and they are causing real problems. Go search
> for reports of Amazon AWS slowdowns.
>
> So this whole "security is so important that performance doesn't
> matter" mindset is pure and utter garbage.
>
> And the whole "normal people won't even notice" is pure garbage too.
> Don't spread that bullshit when you see actual normal people
> complaining.
>
> Performance matters. A *LOT*.

I'm thinking we should provide the option to at least build the
hot-path nospec_array_ptr() usages without an lfence.

    CONFIG_SPECTRE1_PARANOIA_SAFE
    CONFIG_SPECTRE1_PARANOIA_PERF

...if only for easing performance testing and let the distribution set
its policy.

Where hot-path usages can do:

    nospec_relax(nospec_array_ptr())

...to optionally ellide the lfence.