RE: [RFC v2] non-temporal memcpy

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> 
> > From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> > Sent: Wednesday, 10 August 2022 13.56
> >
> > On 2022-08-09 17:26, Stephen Hemminger wrote:
> 
> [...]
> 
> >
> > Alignment seems like a non-issue to me. A NT-store memcpy() can be
> > made free of alignment requirements, incurring only a very slight cost
> > for the always-aligned case (who has their data always 16-byte aligned
> > anyways?).
> >
> > The memory barrier required on x86 seems like a bigger issue.
> >
> > > Maybe rte_non_cache_copy()?
> > >
> >
> > rte_memcpy_nt_weakly_ordered(), or rte_memcpy_nt_weak(). And a
> > rte_memcpy_nt() with the sfence is place, which the user hopefully
> > will find first? I don't know. I would prefer not having the weak
> > variant at all.
I think providing weakly ordered version is required to offset the cost of the barriers. One might be able to copy multiple packets and then issue a barrier.

> >
> > Accepting weak memory ordering (i.e., no sfence) could also be one of
> > the flags, assuming rte_memcpy_nt() would have a flags parameter.
> > Default is safe (=memcpy() semantics), but potentially slower.
> 
> Excellent idea!
> 
> >
> > > Want to avoid the naive user just doing s/memcpy/rte_memcpy_nt/ and
> > expect
> > > everything to work.