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

From: Dan Williams
Date: Fri May 05 2017 - 22:16:13 EST

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.