From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752531AbdEEWZG (ORCPT ); Fri, 5 May 2017 18:25:06 -0400 Received: from mail-oi0-f54.google.com ([209.85.218.54]:36108 "EHLO mail-oi0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751415AbdEEWZE (ORCPT ); Fri, 5 May 2017 18:25:04 -0400 MIME-Version: 1.0 In-Reply-To: <1494016773.30303.69.camel@hpe.com> References: <20170427063054.soejyqocqqrihfdw@gmail.com> <149340820800.28724.16189291963486607562.stgit@dwillia2-desk3.amr.corp.intel.com> <1494016773.30303.69.camel@hpe.com> From: Dan Williams Date: Fri, 5 May 2017 15:25:03 -0700 Message-ID: Subject: Re: [PATCH v2] x86, uaccess: introduce copy_from_iter_wt for pmem / writethrough operations To: "Kani, Toshimitsu" Cc: "viro@zeniv.linux.org.uk" , "linux-kernel@vger.kernel.org" , "linux-block@vger.kernel.org" , "jmoyer@redhat.com" , "tglx@linutronix.de" , "hch@lst.de" , "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" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 5, 2017 at 1:39 PM, Kani, Toshimitsu wrote: > On Fri, 2017-04-28 at 12:39 -0700, Dan Williams wrote: >> The pmem driver has a need to transfer data with a persistent memory >> destination and be able to rely on the fact that the destination >> writes are not cached. It is sufficient for the writes to be flushed >> to a cpu-store-buffer (non-temporal / "movnt" in x86 terms), as we >> expect userspace to call fsync() to ensure data-writes have reached a >> power-fail-safe zone in the platform. The fsync() triggers a REQ_FUA >> or REQ_FLUSH to the pmem driver which will turn around and fence >> previous writes with an "sfence". >> >> Implement a __copy_from_user_inatomic_wt, memcpy_page_wt, and >> memcpy_wt, that guarantee that the destination buffer is not dirty in >> the cpu cache on completion. The new copy_from_iter_wt and sub- >> routines will be used to replace the "pmem api" (include/linux/pmem.h >> + arch/x86/include/asm/pmem.h). The availability of >> copy_from_iter_wt() and memcpy_wt() are gated by the >> CONFIG_ARCH_HAS_UACCESS_WT config symbol, and fallback to >> copy_from_iter_nocache() and plain memcpy() otherwise. >> >> This is meant to satisfy the concern from Linus that if a driver >> wants to do something beyond the normal nocache semantics it should >> be something private to that driver [1], and Al's concern that >> anything uaccess related belongs with the rest of the uaccess code >> [2]. >> >> [1]: https://lists.01.org/pipermail/linux-nvdimm/2017-January/008364. >> html >> [2]: https://lists.01.org/pipermail/linux-nvdimm/2017-April/009942.ht >> ml >> >> Cc: >> Cc: Jan Kara >> Cc: Jeff Moyer >> Cc: Ingo Molnar >> Cc: Christoph Hellwig >> Cc: "H. Peter Anvin" >> Cc: Al Viro >> Cc: Thomas Gleixner >> Cc: Matthew Wilcox >> Cc: Ross Zwisler >> Signed-off-by: Dan Williams >> --- >> 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.