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=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 649EFC433E6 for ; Fri, 12 Mar 2021 16:20:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2C6AC6500B for ; Fri, 12 Mar 2021 16:20:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232116AbhCLQTk (ORCPT ); Fri, 12 Mar 2021 11:19:40 -0500 Received: from ale.deltatee.com ([204.191.154.188]:42376 "EHLO ale.deltatee.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232054AbhCLQTP (ORCPT ); Fri, 12 Mar 2021 11:19:15 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=deltatee.com; s=20200525; h=Subject:In-Reply-To:MIME-Version:Date: Message-ID:From:References:Cc:To:content-disposition; bh=I89VDuZejubSyokWT+qiAjRrhT4T4AjRHHyD/4UUauw=; b=YWISg10aSxm+jQaX+X6OhfxFPO wP4pNsP9huRyLmE2n03E4hyIpdRH1yQeyJ+V9wg1wgsxjmVmS1xL+gBAk6XsaxpB+l0aIYU98JxQZ ciEEug4Y5ay/LYN0UyHg/bEyINwquKoUxnSPKQlqr1ajywPFKVP1ujdzy7/srZYnjWdTlz12JRzIw pI3U2FUOw+RIbjwTV9CMHkfRjKlk6ew/bqxHeIk3ZfYxJ+T4OZozC6OE16gN7hqtn6YLwyH0xRZPH 86uvVT4Ir+w62abwFg8otMV5BX+4owBFgtQVj4Uu2xHzYQCNGi5/KyJe8bEhgCWqEgHXOqoB1UiVm ehwWevcw==; Received: from s01060023bee90a7d.cg.shawcable.net ([24.64.145.4] helo=[192.168.0.10]) by ale.deltatee.com with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1lKkV1-00079p-IQ; Fri, 12 Mar 2021 09:18:52 -0700 To: Robin Murphy , linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-pci@vger.kernel.org, linux-mm@kvack.org, iommu@lists.linux-foundation.org Cc: Minturn Dave B , John Hubbard , Dave Hansen , Ira Weiny , Matthew Wilcox , =?UTF-8?Q?Christian_K=c3=b6nig?= , Jason Gunthorpe , Jason Ekstrand , Daniel Vetter , Dan Williams , Stephen Bates , Jakowski Andrzej , Christoph Hellwig , Xiong Jianxin References: <20210311233142.7900-1-logang@deltatee.com> <6b9be188-1ec7-527c-ae47-3f5b4e153758@arm.com> From: Logan Gunthorpe Message-ID: Date: Fri, 12 Mar 2021 09:18:46 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <6b9be188-1ec7-527c-ae47-3f5b4e153758@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-CA Content-Transfer-Encoding: 7bit X-SA-Exim-Connect-IP: 24.64.145.4 X-SA-Exim-Rcpt-To: jianxin.xiong@intel.com, hch@lst.de, andrzej.jakowski@intel.com, sbates@raithlin.com, dan.j.williams@intel.com, daniel.vetter@ffwll.ch, jason@jlekstrand.net, jgg@ziepe.ca, christian.koenig@amd.com, willy@infradead.org, iweiny@intel.com, dave.hansen@linux.intel.com, jhubbard@nvidia.com, dave.b.minturn@intel.com, iommu@lists.linux-foundation.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, linux-block@vger.kernel.org, linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org, robin.murphy@arm.com X-SA-Exim-Mail-From: logang@deltatee.com Subject: Re: [RFC PATCH v2 00/11] Add support to dma_map_sg for P2PDMA X-SA-Exim-Version: 4.2.1 (built Wed, 08 May 2019 21:11:16 +0000) X-SA-Exim-Scanned: Yes (on ale.deltatee.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-03-12 8:51 a.m., Robin Murphy wrote: > On 2021-03-11 23:31, Logan Gunthorpe wrote: >> Hi, >> >> This is a rework of the first half of my RFC for doing P2PDMA in >> userspace >> with O_DIRECT[1]. >> >> The largest issue with that series was the gross way of flagging P2PDMA >> SGL segments. This RFC proposes a different approach, (suggested by >> Dan Williams[2]) which uses the third bit in the page_link field of the >> SGL. >> >> This approach is a lot less hacky but comes at the cost of adding a >> CONFIG_64BIT dependency to CONFIG_PCI_P2PDMA and using up the last >> scarce bit in the page_link. For our purposes, a 64BIT restriction is >> acceptable but it's not clear if this is ok for all usecases hoping >> to make use of P2PDMA. >> >> Matthew Wilcox has already suggested (off-list) that this is the wrong >> approach, preferring a new dma mapping operation and an SGL >> replacement. I >> don't disagree that something along those lines would be a better long >> term solution, but it involves overcoming a lot of challenges to get >> there. Creating a new mapping operation still means adding support to >> more >> than 25 dma_map_ops implementations (many of which are on obscure >> architectures) or creating a redundant path to fallback with dma_map_sg() >> for every driver that uses the new operation. This RFC is an approach >> that doesn't require overcoming these blocks. > > I don't really follow that argument - you're only adding support to two > implementations with the awkward flag, so why would using a dedicated > operation instead be any different? Whatever callers need to do if > dma_pci_p2pdma_supported() says no, they could equally do if > dma_map_p2p_sg() (or whatever) returns -ENXIO, no? The thing is if the dma_map_sg doesn't support P2PDMA then P2PDMA transactions cannot be done, but regular transactions can still go through as they always did. But replacing dma_map_sg() with dma_map_new() affects all operations, P2PDMA or otherwise. If dma_map_new() isn't supported it can't simply not support P2PDMA; it has to maintain a fallback path to dma_map_sg(). Given that the inputs and outputs for dma_map_new() will be completely different data structures this will be quite a lot of similar paths required in the driver. (ie mapping a bvec to the input struct and the output struct to hardware requirements) If a bug crops up in the old dma_map_sg(), developers might not notice it for some time seeing it won't be used on the most popular architectures. Logan