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

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Date:   Sat, 6 Jan 2018 17:08:58 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Network Development <netdev@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 01/18] asm-generic/barrier: add generic nospec helpers

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@...ux-foundation.org> wrote:
> > On Fri, Jan 5, 2018 at 5:09 PM, Dan Williams <dan.j.williams@...el.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.