Re: [PATCH 4/6] arm64/io: Provide a WC friendly __iowriteXX_copy()

Catalin Marinas
2024-02-29T10:24:52Z

On Wed, Feb 28, 2024 at 07:06:16PM -0400, Jason Gunthorpe wrote:
> On Tue, Feb 27, 2024 at 10:37:18AM +0000, Catalin Marinas wrote:
> > On Tue, Feb 20, 2024 at 09:17:08PM -0400, Jason Gunthorpe wrote:
> > > +/*
> > > + * This generates a memcpy that works on a from/to address which is aligned to
> > > + * bits. Count is in terms of the number of bits sized quantities to copy. It
> > > + * optimizes to use the STR groupings when possible so that it is WC friendly.
> > > + */
> > > +#define memcpy_toio_aligned(to, from, count, bits) \
> > > + ({ \
> > > + volatile u##bits __iomem *_to = to; \
> > > + const u##bits *_from = from; \
> > > + size_t _count = count; \
> > > + const u##bits *_end_from = _from + ALIGN_DOWN(_count, 8); \
> > > + \
> > > + for (; _from < _end_from; _from += 8, _to += 8) \
> > > + __const_memcpy_toio_aligned##bits(_to, _from, 8); \
> > > + if ((_count % 8) >= 4) { \
> > > + __const_memcpy_toio_aligned##bits(_to, _from, 4); \
> > > + _from += 4; \
> > > + _to += 4; \
> > > + } \
> > > + if ((_count % 4) >= 2) { \
> > > + __const_memcpy_toio_aligned##bits(_to, _from, 2); \
> > > + _from += 2; \
> > > + _to += 2; \
> > > + } \
> > > + if (_count % 2) \
> > > + __const_memcpy_toio_aligned##bits(_to, _from, 1); \
> > > + })
> >
> > Do we actually need all this if count is not constant? If it's not
> > performance critical anywhere, I'd rather copy the generic
> > implementation, it's easier to read.
>
> Which generic version?

The current __iowriteXX_copy() in lib/iomap_copy.c (copy them over or
add some preprocessor reuse the generic functions).

> The point is to maximize WC effects with non-constant values, so I
> think we do need something like this. ie we can't just fall back to
> looping over 64 bit stores one at a time.

If that's a case you are also targeting and have seen it in practice,
that's fine. But I had the impression that you are mostly after the
constant count case which is already addressed by the other part of this
patch. For the non-constant case, we have a DGH only at the end of
whatever buffer was copied rather than after every 64-byte increments
you'd get for a count of 8.

> Most places I know about using this are performance paths, the entire
> iocopy infrastructure was introduced as an x86 performance
> optimization..

At least the x86 case makes sense even from a maintenance perspective,
it's just a much simpler "rep movsl". I just want to make sure we don't
over-complicate this code on arm64 unnecessarily.

--
Catalin