netdev - Re: [PATCH 06/18] x86, barrier: stop speculation for failed access

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Date:   Sun, 7 Jan 2018 18:24:11 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Willy Tarreau <w@....eu>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Network Development <netdev@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

On Sun, Jan 07, 2018 at 12:15:40PM -0800, Dan Williams wrote:
> 
> 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

SAFE vs PERF naming is problematic and misleading, since users don't
have the data to make a decision they will be forced to go with SAFE.

What is not safe about array_access() macro with AND ?
How lfence approach makes it safer ?
Only because lfence was blessed by intel earlier when
they couldn't figure out a different way?

How about:
CONFIG_SPECTRE1_WORKAROUND_INDEX_MASK
CONFIG_SPECTRE1_WORKAROUND_LOAD_FENCE

> ...if only for easing performance testing and let the distribution set
> its policy.
> 
> Where hot-path usages can do:
> 
>     nospec_relax(nospec_array_ptr())

AND approach doesn't prevent speculation hence nospec_ is an incorrect prefix.
Alan's "speculation management" terminology fits well here.

Can we keep array_access() name and change it underneath to
either mask or lfence ?