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

[Dan Williams]

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.