prefetch inconsistency in copy_page()

[Pinski, Andrew]

> -----Original Message-----
> From: Will Deacon [mailto:will.deacon at arm.com]
> Sent: Tuesday, July 11, 2017 10:53 AM
> To: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Cc: Mark Rutland <mark.rutland at arm.com>; Pinski, Andrew
> <Andrew.Pinski at cavium.com>; linux-arm-kernel at lists.infradead.org
> Subject: Re: prefetch inconsistency in copy_page()
> 
> On Tue, Jul 11, 2017 at 10:42:19AM +0100, Ard Biesheuvel wrote:
> > Hi all,
> >
> > No big deal, but I spotted an inconsistency in how copy_page() does
> > its prefetching. With the code as-is, it prefetches 2 lines at the
> > start of the function, but it is off by one line in the loop, i.e., it
> > prefetches 3 lines ahead (and skips a line at entry). So imo, we need
> > either
> >
> > """
> > @@ -30,9 +30,10 @@
> >   */
> >  ENTRY(copy_page)
> >  alternative_if ARM64_HAS_NO_HW_PREFETCH
> > -       # Prefetch two cache lines ahead.
> > +       # Prefetch three cache lines ahead.
> >         prfm    pldl1strm, [x1, #128]
> >         prfm    pldl1strm, [x1, #256]
> > +       prfm    pldl1strm, [x1, #384]
> >  alternative_else_nop_endif
> >
> >         ldp     x2, x3, [x1]
> > """
> >
> > or
> >
> > """
> > @@ -50,7 +51,7 @@ alternative_else_nop_endif
> >         subs    x18, x18, #128
> >
> >  alternative_if ARM64_HAS_NO_HW_PREFETCH
> > -       prfm    pldl1strm, [x1, #384]
> > +       prfm    pldl1strm, [x1, #256]
> >  alternative_else_nop_endif
> >
> >         stnp    x2, x3, [x0]
> >
> > """
> >
> > to make things consistent again.
> >
> > Thoughts?
> 
> Either of those look good to me. Andrew -- any preference? If not, I'll
> take the second one.

The first one is my preference but both are ok to me.

Thanks,
Andrew

> 
> Will