Re: [PATCH 01/18] asm-generic/barrier: add generic nospec helpers

Dan Williams
2018-01-06T05:23:09Z

On Fri, Jan 5, 2018 at 6:55 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Jan 5, 2018 at 5:09 PM, Dan Williams <dan.j.williams@intel.com> wrote:
>> +#ifndef nospec_ptr
>> +#define nospec_ptr(ptr, lo, hi) \
>
> Do we actually want this horrible interface?
>
> It just causes the compiler - or inline asm - to generate worse code,
> because it needs to compare against both high and low limits.
>
> Basically all users are arrays that are zero-based, and where a
> comparison against the high _index_ limit would be sufficient.
>
> But the way this is all designed, it's literally designed for bad code
> generation for the unusual case, and the usual array case is written
> in the form of the unusual and wrong non-array case. That really seems
> excessively stupid.

Yes, it appears we can kill nospec_ptr() and move nospec_array_ptr()
to assume 0 based arrays rather than use nospec_ptr.