Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

Alexei Starovoitov
2018-01-08T02:24:14Z

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 ?