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=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 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 7859EC433E0 for ; Fri, 12 Mar 2021 15:51:50 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 2179A64F72 for ; Fri, 12 Mar 2021 15:51:50 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2179A64F72 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id AABF76B0072; Fri, 12 Mar 2021 10:51:49 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id A7FC36B0073; Fri, 12 Mar 2021 10:51:49 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8FBFF6B0074; Fri, 12 Mar 2021 10:51:49 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0173.hostedemail.com [216.40.44.173]) by kanga.kvack.org (Postfix) with ESMTP id 70DF06B0072 for ; Fri, 12 Mar 2021 10:51:49 -0500 (EST) Received: from smtpin23.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 32EEE8417 for ; Fri, 12 Mar 2021 15:51:49 +0000 (UTC) X-FDA: 77911662738.23.471995F Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf19.hostedemail.com (Postfix) with ESMTP id A57CF90009EE for ; Fri, 12 Mar 2021 15:51:45 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 79DA01FB; Fri, 12 Mar 2021 07:51:47 -0800 (PST) Received: from [10.57.52.136] (unknown [10.57.52.136]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 38D593F7D7; Fri, 12 Mar 2021 07:51:43 -0800 (PST) Subject: Re: [RFC PATCH v2 00/11] Add support to dma_map_sg for P2PDMA To: Logan Gunthorpe , 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> From: Robin Murphy Message-ID: <6b9be188-1ec7-527c-ae47-3f5b4e153758@arm.com> Date: Fri, 12 Mar 2021 15:51:37 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <20210311233142.7900-1-logang@deltatee.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit X-Stat-Signature: 9adzj9czwoqiguxtyrczyjqts788ygr9 X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: A57CF90009EE Received-SPF: none (arm.com>: No applicable sender policy available) receiver=imf19; identity=mailfrom; envelope-from=""; helo=foss.arm.com; client-ip=217.140.110.172 X-HE-DKIM-Result: none/none X-HE-Tag: 1615564305-11847 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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? We don't try to multiplex .map_resource through .map_page, so there doesn't seem to be any good reason to force that complexity on .map_sg either. And having a distinct API from the outset should make it a lot easier to transition to better "list of P2P memory regions" data structures in future without rewriting the whole world. As it is, there are potential benefits in a P2P interface which can define its own behaviour - for instance if threw out the notion of segment merging it could save a load of bother by just maintaining the direct correlation between pages and DMA addresses. Robin. > Any alternative ideas or feedback is welcome. > > These patches are based on v5.12-rc2 and a git branch is available here: > > https://github.com/sbates130272/linux-p2pmem/ p2pdma_dma_map_ops_rfc > > A branch with the patches from the previous RFC that add userspace > O_DIRECT support is available at the same URL with the name > "p2pdma_dma_map_ops_rfc+user" (however, none of the issues with those > extra patches from the feedback of the last posting have been fixed). > > Thanks, > > Logan > > [1] https://lore.kernel.org/linux-block/20201106170036.18713-1-logang@deltatee.com/ > [2] https://lore.kernel.org/linux-block/CAPcyv4ifGcrdOtUt8qr7pmFhmecGHqGVre9G0RorGczCGVECQQ@mail.gmail.com/ > > -- > > Logan Gunthorpe (11): > PCI/P2PDMA: Pass gfp_mask flags to upstream_bridge_distance_warn() > PCI/P2PDMA: Avoid pci_get_slot() which sleeps > PCI/P2PDMA: Attempt to set map_type if it has not been set > PCI/P2PDMA: Introduce pci_p2pdma_should_map_bus() and > pci_p2pdma_bus_offset() > lib/scatterlist: Add flag for indicating P2PDMA segments in an SGL > dma-direct: Support PCI P2PDMA pages in dma-direct map_sg > dma-mapping: Add flags to dma_map_ops to indicate PCI P2PDMA support > iommu/dma: Support PCI P2PDMA pages in dma-iommu map_sg > block: Add BLK_STS_P2PDMA > nvme-pci: Check DMA ops when indicating support for PCI P2PDMA > nvme-pci: Convert to using dma_map_sg for p2pdma pages > > block/blk-core.c | 2 + > drivers/iommu/dma-iommu.c | 63 +++++++++++++++++++++----- > drivers/nvme/host/core.c | 3 +- > drivers/nvme/host/nvme.h | 2 +- > drivers/nvme/host/pci.c | 38 +++++++--------- > drivers/pci/Kconfig | 2 +- > drivers/pci/p2pdma.c | 89 +++++++++++++++++++++++++++++++------ > include/linux/blk_types.h | 7 +++ > include/linux/dma-map-ops.h | 3 ++ > include/linux/dma-mapping.h | 5 +++ > include/linux/pci-p2pdma.h | 11 +++++ > include/linux/scatterlist.h | 49 ++++++++++++++++++-- > kernel/dma/direct.c | 35 +++++++++++++-- > kernel/dma/mapping.c | 21 +++++++-- > 14 files changed, 271 insertions(+), 59 deletions(-) > > > base-commit: a38fd8748464831584a19438cbb3082b5a2dab15 > -- > 2.20.1 > _______________________________________________ > iommu mailing list > iommu@lists.linux-foundation.org > https://lists.linuxfoundation.org/mailman/listinfo/iommu >