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=-5.3 required=3.0 tests=BAYES_00, 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 2E47AC43381 for ; Fri, 12 Mar 2021 17:47:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 097AB64F0D for ; Fri, 12 Mar 2021 17:47:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232804AbhCLRrJ (ORCPT ); Fri, 12 Mar 2021 12:47:09 -0500 Received: from foss.arm.com ([217.140.110.172]:58176 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231557AbhCLRqo (ORCPT ); Fri, 12 Mar 2021 12:46:44 -0500 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 EA4DC1FB; Fri, 12 Mar 2021 09:46:43 -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 74DF03F7D7; Fri, 12 Mar 2021 09:46:38 -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> <6b9be188-1ec7-527c-ae47-3f5b4e153758@arm.com> From: Robin Murphy Message-ID: <90a2825c-da2f-c031-a70f-08c5efb3db56@arm.com> Date: Fri, 12 Mar 2021 17:46:31 +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: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-03-12 16:18, Logan Gunthorpe wrote: > > > 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(). But AFAICS the equivalent fallback path still has to exist either way. My impression so far is that callers would end up looking something like this: if (dma_pci_p2pdma_supported()) { if (dma_map_sg(...) < 0) //do non-p2p fallback due to p2p failure } else { //do non-p2p fallback due to lack of support } at which point, simply: if (dma_map_sg_p2p(...) < 0) //do non-p2p fallback either way seems entirely reasonable. What am I missing? Let's not pretend that overloading an existing API means we can start feeding P2P pages into any old subsystem/driver without further changes - there already *are* at least some that retry ad infinitum if DMA mapping fails (the USB layer springs to mind...) and thus wouldn't handle the PCI_P2PDMA_MAP_NOT_SUPPORTED case acceptably. > 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. Huh? I'm specifically suggesting a new interface that takes the *same* data structure (at least to begin with), but just gives us more flexibility in terms of introducing p2p-aware behaviour somewhat more safely. Yes, we already know that we ultimately want something better than scatterlists for representing things like this and dma-buf imports, but that hardly has to happen overnight. Robin.