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

Mark Rutland
2018-01-06T17:09:08Z

On Fri, Jan 05, 2018 at 09:23:06PM -0800, Dan Williams wrote:
> 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.

Sounds good to me; I can respin the arm/arm64 implementations accordingly.

We can always revisit that if we have non-array cases that need to be covered.

Thanks,
Mark.