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=-7.6 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,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 6B6BEC433F5 for ; Tue, 7 Sep 2021 09:43:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4989660F6D for ; Tue, 7 Sep 2021 09:43:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241890AbhIGJoU (ORCPT ); Tue, 7 Sep 2021 05:44:20 -0400 Received: from foss.arm.com ([217.140.110.172]:33852 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242700AbhIGJoS (ORCPT ); Tue, 7 Sep 2021 05:44:18 -0400 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 995D131B; Tue, 7 Sep 2021 02:43:12 -0700 (PDT) Received: from [10.57.15.112] (unknown [10.57.15.112]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7B47C3F766; Tue, 7 Sep 2021 02:43:11 -0700 (PDT) Subject: Re: [question] Assign multiple devices from different SMMUs to a arm_smmu_domain To: Kunkun Jiang , Will Deacon , Linux IOMMU , open list Cc: wanghaibin.wang@huawei.com, Zenghui Yu , Keqian Zhu References: From: Robin Murphy Message-ID: <202f32cd-8036-563e-028b-781b999766be@arm.com> Date: Tue, 7 Sep 2021 10:43:05 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-09-07 08:41, Kunkun Jiang wrote: > Hi all, > > I am working on VFIO DMA dirty pages tracking based on ARM SMMU HTTU, > and have done a lot of testing.In the test, I found a problem that > greatly affects > performance of VFIO DMA dirty pages tracking. > > According to the current arm-smmu-v3 driver, multiple VFIO pass-through > device comes from different SMMUs will be assigned to different > arm_smmu_domain. It will create page table for each arm_smmu_domain, > even though these page tables are exactly the same. Bacause dirty pages > tracking needs to traverse the page table, multiple page tables will make > performance worse. > > I learned the ARM SMMUv3 spec and had some exchanges with my colleagues > who work on SMMU hardware. I did not find the restriction that multiple > SMMUs cannot share the same page table. We migth be able to do this like > x86 IOMMU. If I have missed something, please point it out. Sure, it's not impossible, there are just a lot of fiddly considerations, mostly around how to handle SMMU instances with different capabilities. We haven't had a strong need to support this case so far, so we've simply been avoiding all that complexity. Robin. > Looking forward to your suggestions.😁 > > Thanks, > Kunkun Jiang >