linux-kernel - Re: [PATCH v2 1/3] introduce memcpy

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Date:   Fri, 30 Dec 2016 03:52:52 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Boaz Harrosh <boaz@...xistor.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "Moreno, Oliver" <oliver.moreno@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "boylston@...romesa.net" <boylston@...romesa.net>
Subject: Re: [PATCH v2 1/3] introduce memcpy_nocache()

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?