From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f196.google.com ([209.85.128.196]:34589 "EHLO mail-wr0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750984AbdEGWYP (ORCPT ); Sun, 7 May 2017 18:24:15 -0400 Date: Sun, 7 May 2017 10:57:30 +0200 From: Ingo Molnar To: Dan Williams Cc: "Kani, Toshimitsu" , "linux-kernel@vger.kernel.org" , "linux-block@vger.kernel.org" , "jmoyer@redhat.com" , "tglx@linutronix.de" , "hch@lst.de" , "viro@zeniv.linux.org.uk" , "x86@kernel.org" , "mawilcox@microsoft.com" , "hpa@zytor.com" , "linux-nvdimm@lists.01.org" , "mingo@redhat.com" , "linux-fsdevel@vger.kernel.org" , "ross.zwisler@linux.intel.com" , "jack@suse.cz" Subject: Re: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem / writethrough operations Message-ID: <20170507085730.ykn5abjol5o66ond@gmail.com> References: <20170427063054.soejyqocqqrihfdw@gmail.com> <149340820800.28724.16189291963486607562.stgit@dwillia2-desk3.amr.corp.intel.com> <1494016773.30303.69.camel@hpe.com> <1494024273.30303.71.camel@hpe.com> <20170506094652.sgiwijzursabbiab@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-ID: * Dan Williams wrote: > On Sat, May 6, 2017 at 2:46 AM, Ingo Molnar wrote: > > > > * Dan Williams wrote: > > > >> On Fri, May 5, 2017 at 3:44 PM, Kani, Toshimitsu wrote: > >> > On Fri, 2017-05-05 at 15:25 -0700, Dan Williams wrote: > >> >> On Fri, May 5, 2017 at 1:39 PM, Kani, Toshimitsu > >> >> 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