Re: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem / writethrough operations

[Ingo Molnar]

* Dan Williams <dan.j.williams@xxxxxxxxx> wrote:

> On Sat, May 6, 2017 at 2:46 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> >> On Fri, May 5, 2017 at 3:44 PM, Kani, Toshimitsu <toshi.kani@xxxxxxx> wrote:
> >> > On Fri, 2017-05-05 at 15:25 -0700, Dan Williams wrote:
> >> >> On Fri, May 5, 2017 at 1:39 PM, Kani, Toshimitsu <toshi.kani@xxxxxxx>
> >> >> wrote:
> >> >  :
> >> >> > > ---
> >> >> > > Changes since the initial RFC:
> >> >> > > * s/writethru/wt/ since we already have ioremap_wt(),
> >> >> > > set_memory_wt(), etc. (Ingo)
> >> >> >
> >> >> > Sorry I should have said earlier, but I think the term "wt" is
> >> >> > misleading.  Non-temporal stores used in memcpy_wt() provide WC
> >> >> > semantics, not WT semantics.
> >> >>
> >> >> The non-temporal stores do, but memcpy_wt() is using a combination of
> >> >> non-temporal stores and explicit cache flushing.
> >> >>
> >> >> > How about using "nocache" as it's been
> >> >> > used in __copy_user_nocache()?
> >> >>
> >> >> The difference in my mind is that the "_nocache" suffix indicates
> >> >> opportunistic / optional cache pollution avoidance whereas "_wt"
> >> >> strictly arranges for caches not to contain dirty data upon
> >> >> completion of the routine. For example, non-temporal stores on older
> >> >> x86 cpus could potentially leave dirty data in the cache, so
> >> >> memcpy_wt on those cpus would need to use explicit cache flushing.
> >> >
> >> > I see.  I agree that its behavior is different from the existing one
> >> > with "_nocache".   That said, I think "wt" or "write-through" generally
> >> > means that writes allocate cachelines and keep them clean by writing to
> >> > memory.  So, subsequent reads to the destination will hit the
> >> > cachelines.  This is not the case with this interface.
> >>
> >> True... maybe _nocache_strict()? Or, leave it _wt() until someone
> >> comes along and is surprised that the cache is not warm for reads
> >> after memcpy_wt(), at which point we can ask "why not just use plain
> >> memcpy then?", or set the page-attributes to WT.
> >
> > Perhaps a _nocache_flush() postfix, to signal both that it's non-temporal and that
> > no cache line is left around afterwards (dirty or clean)?
> 
> Yes, I think "flush" belongs in the name, and to make it easily
> grep-able separate from _nocache we can call it _flushcache? An
> efficient implementation will use _nocache / non-temporal stores
> internally, but external consumers just care about the state of the
> cache after the call.

_flushcache() works for me too.

Thanks,

	Ingo