From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 53F4BC43144 for ; Mon, 25 Jun 2018 16:12:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 074A425DEC for ; Mon, 25 Jun 2018 16:12:18 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 074A425DEC Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=ACULAB.COM Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753556AbeFYQMQ convert rfc822-to-8bit (ORCPT ); Mon, 25 Jun 2018 12:12:16 -0400 Received: from smtp-out6.electric.net ([192.162.217.181]:51103 "EHLO smtp-out6.electric.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752224AbeFYQMO (ORCPT ); Mon, 25 Jun 2018 12:12:14 -0400 Received: from 1fXU5z-0009FV-US by out6d.electric.net with emc1-ok (Exim 4.90_1) (envelope-from ) id 1fXU60-0009Kw-W9; Mon, 25 Jun 2018 09:12:04 -0700 Received: by emcmailer; Mon, 25 Jun 2018 09:12:04 -0700 Received: from [156.67.243.126] (helo=AcuMS.aculab.com) by out6d.electric.net with esmtps (TLSv1.2:ECDHE-RSA-AES256-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1fXU5z-0009FV-US; Mon, 25 Jun 2018 09:12:03 -0700 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 25 Jun 2018 17:13:42 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Mon, 25 Jun 2018 17:13:42 +0100 From: David Laight To: 'Fenghua Yu' , Thomas Gleixner CC: Ingo Molnar , H Peter Anvin , Ashok Raj , Alan Cox , Ravi V Shankar , linux-kernel , x86 Subject: RE: [RFC PATCH 1/8] x86/cpufeatures: Enumerate MOVDIRI instruction Thread-Topic: [RFC PATCH 1/8] x86/cpufeatures: Enumerate MOVDIRI instruction Thread-Index: AQHUCBXina7mb9UEzUeoseVjbd5RvqRxLHeg Date: Mon, 25 Jun 2018 16:13:42 +0000 Message-ID: References: <1529118375-90191-1-git-send-email-fenghua.yu@intel.com> <1529118375-90191-2-git-send-email-fenghua.yu@intel.com> <20180619213644.GC112652@romley-ivt3.sc.intel.com> In-Reply-To: <20180619213644.GC112652@romley-ivt3.sc.intel.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.33] Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 X-Outbound-IP: 156.67.243.126 X-Env-From: David.Laight@ACULAB.COM X-Proto: esmtps X-Revdns: X-HELO: AcuMS.aculab.com X-TLS: TLSv1.2:ECDHE-RSA-AES256-SHA384:256 X-Authenticated_ID: X-PolicySMART: 3396946, 3397078 X-Virus-Status: Scanned by VirusSMART (c) X-Virus-Status: Scanned by VirusSMART (s) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Fenghua Yu > Sent: 19 June 2018 22:37 > To: Thomas Gleixner > Cc: Fenghua Yu; Ingo Molnar; H Peter Anvin; Ashok Raj; Alan Cox; Ravi V Shankar; linux-kernel; x86 > Subject: Re: [RFC PATCH 1/8] x86/cpufeatures: Enumerate MOVDIRI instruction > > On Tue, Jun 19, 2018 at 10:57:44AM +0200, Thomas Gleixner wrote: > > On Fri, 15 Jun 2018, Fenghua Yu wrote: > > > > > MOVDIRI moves doubleword or quadword from register to memory through > > > direct store which is implemented by using write combining (WC) for > > > writing data directly into memory without caching the data. > > > > And that is useful for what? > > Programmable agents can handle streaming offload (e.g. high speed packet > processing in network). Hardware implements a doorbell (tail pointer) > register that is updated by software when adding new work-elements to > the streaming offload work-queue. > > MOVDIRI can be used as the doorbell write which is a 4-byte or 8-byte > uncachable write to MMIO. MOVDIRI has lower overhead than other ways > to write the doorbell. I'd have thought that it wouldn't make any significant difference for uncached accesses to device registers. > In low latency offload (e.g. Non-Volatile Memory, etc), MOVDIR64B writes > work descriptors (and data in some cases) to device-hosted work-queues > with atomicity. More likely it is useful more writing to memory without polluting the data cache. This might be because the programmer knows the data won't be read for a long time (at least by the cpu in question). It might also be useful to avoid a lot of cache snooping on data that will be accessed by hardware - especially if the hardware is also likely to be writing to the same cache line. I can also just about imagine MOVDIR64B being useful for generating 64 byte PCIe TLP to optimise memcpy_to/fromio() without needing an AVX512 register. David