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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, 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 C37EEC2BA19 for ; Mon, 6 Apr 2020 11:50:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A1E62206F8 for ; Mon, 6 Apr 2020 11:50:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727732AbgDFLuW (ORCPT ); Mon, 6 Apr 2020 07:50:22 -0400 Received: from verein.lst.de ([213.95.11.211]:33172 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727515AbgDFLuW (ORCPT ); Mon, 6 Apr 2020 07:50:22 -0400 Received: by verein.lst.de (Postfix, from userid 2407) id 35D0268C4E; Mon, 6 Apr 2020 13:50:17 +0200 (CEST) Date: Mon, 6 Apr 2020 13:50:16 +0200 From: Christoph Hellwig To: Alexey Kardashevskiy Cc: Christoph Hellwig , "Aneesh Kumar K.V" , iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org, Lu Baolu , Greg Kroah-Hartman , Joerg Roedel , Robin Murphy , linux-kernel@vger.kernel.org, Michael Ellerman Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct device Message-ID: <20200406115016.GA10941@lst.de> References: <20200323083705.GA31245@lst.de> <20200323085059.GA32528@lst.de> <87sghz2ibh.fsf@linux.ibm.com> <20200323172256.GB31269@lst.de> <20200324075402.GJ23447@lst.de> <41975da3-3a4a-fc3c-2b90-8d607cf220e6@ozlabs.ru> <20200325083740.GC21605@lst.de> <213b0c7d-f908-b4f4-466d-6240c3622cd6@ozlabs.ru> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <213b0c7d-f908-b4f4-466d-6240c3622cd6@ozlabs.ru> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 03, 2020 at 07:38:11PM +1100, Alexey Kardashevskiy wrote: > > > On 26/03/2020 12:26, Alexey Kardashevskiy wrote: > > > > > > On 25/03/2020 19:37, Christoph Hellwig wrote: > >> On Wed, Mar 25, 2020 at 03:51:36PM +1100, Alexey Kardashevskiy wrote: > >>>>> This is for persistent memory which you can DMA to/from but yet it does > >>>>> not appear in the system as a normal memory and therefore requires > >>>>> special handling anyway (O_DIRECT or DAX, I do not know the exact > >>>>> mechanics). All other devices in the system should just run as usual, > >>>>> i.e. use 1:1 mapping if possible. > >>>> > >>>> On other systems (x86 and arm) pmem as long as it is page backed does > >>>> not require any special handling. This must be some weird way powerpc > >>>> fucked up again, and I suspect you'll have to suffer from it. > >>> > >>> > >>> It does not matter if it is backed by pages or not, the problem may also > >>> appear if we wanted for example p2p PCI via IOMMU (between PHBs) and > >>> MMIO might be mapped way too high in the system address space and make > >>> 1:1 impossible. > >> > >> How can it be mapped too high for a direct mapping with a 64-bit DMA > >> mask? > > > > The window size is limited and often it is not even sparse. It requires > > an 8 byte entry per an IOMMU page (which is most commonly is 64k max) so > > 1TB limit (a guest RAM size) is a quite real thing. MMIO is mapped to > > guest physical address space outside of this 1TB (on PPC). > > > > > > I am trying now this approach on top of yours "dma-bypass.3" (it is > "wip", needs an upper boundary check): > > https://github.com/aik/linux/commit/49d73c7771e3f6054804f6cfa80b4e320111662d > > Do you see any serious problem with this approach? Thanks! Do you have a link to the whole branch? The github UI is unfortunately unusable for that (or I'm missing something).