Re: [PATCH v2 1/3] introduce memcpy_nocache()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Dec 29, 2016 at 10:23:15AM -0800, Dan Williams wrote:

> > BTW, your "it's iovec, only non-temporal stores there" logics in
> > arch_copy_from_iter_pmem() is simply wrong - for one thing, unaligned
> > copies will have parts done via normal stores, for another 32bit will
> > _not_ go for non-caching codepath for short copies.  What semantics do
> > we really need there?
> 
> For typical pmem platforms we need to make sure all the writes are on
> the way to memory such than a later sfence can guarantee that all
> previous writes are visible to the platform "ADR" logic.  ADR handles
> flushing memory controller write buffers to media. At a minimum
> arch_copy_from_iter_pmem() needs to trigger a clwb (unordered cache
> line writeback) of each touched cache line if it is not using a cache
> bypassing store.

Um...  Then we do have a problem - nocache variant of uaccess primitives
does *not* guarantee that clwb is redundant.

What about the requirements of e.g. tcp_sendmsg() with its use of
skb_add_data_nocache()?  What warranties do we need there?