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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 E605FC3A5A6 for ; Thu, 19 Sep 2019 09:38:52 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id BC4E321907 for ; Thu, 19 Sep 2019 09:38:52 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BC4E321907 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:41392 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iAstn-0002aP-Uh for qemu-devel@archiver.kernel.org; Thu, 19 Sep 2019 05:38:51 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:45206) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iAst6-00028Z-HV for qemu-devel@nongnu.org; Thu, 19 Sep 2019 05:38:10 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iAst5-0008Vq-1H for qemu-devel@nongnu.org; Thu, 19 Sep 2019 05:38:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59656) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iAst4-0008V6-O9 for qemu-devel@nongnu.org; Thu, 19 Sep 2019 05:38:06 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 822F18665A; Thu, 19 Sep 2019 09:38:05 +0000 (UTC) Received: from [10.72.12.81] (ovpn-12-81.pek2.redhat.com [10.72.12.81]) by smtp.corp.redhat.com (Postfix) with ESMTP id CCB271001B01; Thu, 19 Sep 2019 09:37:56 +0000 (UTC) To: "Tian, Kevin" , "Zhao, Yan Y" References: <60110ea3-9228-7e5d-ea32-05c72a95af0b@redhat.com> <8302a4ae-1914-3046-b3b5-b3234d7dda02@redhat.com> <6d73572e-1e89-b04a-bdd6-98ac73798083@redhat.com> <204219fa-ee72-ca60-52a4-fb4bbc887773@redhat.com> <20190919052819.GA18391@joy-OptiPlex-7040> <7b6d6343-33de-ebd7-9846-af54a45a82a2@redhat.com> <20190919061756.GB18391@joy-OptiPlex-7040> From: Jason Wang Message-ID: <7906030a-00e3-012d-da4f-bb0c1b2901b2@redhat.com> Date: Thu, 19 Sep 2019 17:37:48 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Thu, 19 Sep 2019 09:38:05 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] vhost, iova, and dirty page tracking X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , 'Alex Williamson' , "qemu-devel@nongnu.org" , "Michael S. Tsirkin" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2019/9/19 =E4=B8=8B=E5=8D=883:16, Tian, Kevin wrote: > +Paolo to help clarify here. > >> From: Jason Wang [mailto:jasowang@redhat.com] >> Sent: Thursday, September 19, 2019 2:32 PM >> >> >> On 2019/9/19 =E4=B8=8B=E5=8D=882:17, Yan Zhao wrote: >>> On Thu, Sep 19, 2019 at 02:09:53PM +0800, Jason Wang wrote: >>>> On 2019/9/19 =E4=B8=8B=E5=8D=881:28, Yan Zhao wrote: >>>>> On Thu, Sep 19, 2019 at 09:05:12AM +0800, Jason Wang wrote: >>>>>> On 2019/9/18 =E4=B8=8B=E5=8D=884:37, Tian, Kevin wrote: >>>>>>>> From: Jason Wang [mailto:jasowang@redhat.com] >>>>>>>> Sent: Wednesday, September 18, 2019 2:10 PM >>>>>>>> >>>>>>>>>> Note that the HVA to GPA mapping is not an 1:1 mapping. One >> HVA >>>>>>>> range >>>>>>>>>> could be mapped to several GPA ranges. >>>>>>>>> This is fine. Currently vfio_dma maintains IOVA->HVA mapping. >>>>>>>>> >>>>>>>>> btw under what condition HVA->GPA is not 1:1 mapping? I didn't >> realize it. >>>>>>>> I don't remember the details e.g memory region alias? And neithe= r >> kvm >>>>>>>> nor kvm API does forbid this if my memory is correct. >>>>>>>> >>>>>>> I checked https://qemu.weilnetz.de/doc/devel/memory.html, which >>>>>>> provides an example of aliased layout. However, its aliasing is a= ll >>>>>>> 1:1, instead of N:1. From guest p.o.v every writable GPA implies = an >>>>>>> unique location. Why would we hit the situation where multiple >>>>>>> write-able GPAs are mapped to the same HVA (i.e. same physical >>>>>>> memory location)? >>>>>> I don't know, just want to say current API does not forbid this. S= o we >>>>>> probably need to take care it. >>>>>> >>>>> yes, in KVM API level, it does not forbid two slots to have the sam= e >> HVA(slot->userspace_addr). >>>>> But >>>>> (1) there's only one kvm instance for each vm for each qemu process= . >>>>> (2) all ramblock->host (corresponds to HVA and slot->userspace_addr= ) >> in one qemu >>>>> process is non-overlapping as it's obtained from mmmap(). >>>>> (3) qemu ensures two kvm slots will not point to the same section o= f >> one ramblock. >>>>> So, as long as kvm instance is not shared in two processes, and >>>>> there's no bug in qemu, we can assure that HVA to GPA is 1:1. >>>> Well, you leave this API for userspace, so you can't assume qemu is = the >>>> only user or any its behavior. If you had you should limit it in the= API >>>> level instead of open window for them. >>>> >>>> >>>>> But even if there are two processes operating on the same kvm >> instance >>>>> and manipulating on memory slots, adding an extra GPA along side >> current >>>>> IOVA & HVA to ioctl VFIO_IOMMU_MAP_DMA can still let driver knows >> the >>>>> right IOVA->GPA mapping, right? >>>> It looks fragile. Consider HVA was mapped to both GPA1 and GPA2. >> Guest >>>> maps IOVA to GPA2, so we have IOVA GPA2 HVA in the new ioctl and >> then >>>> log through GPA2. If userspace is trying to sync through GPA1, it wi= ll >>>> miss the dirty page. So for safety we need log both GPA1 and GPA2. (= See >>>> what has been done in log_write_hva() in vhost.c). The only way to d= o >>>> that is to maintain an independent HVA to GPA mapping like what KVM >> or >>>> vhost did. >>>> >>> why GPA1 and GPA2 should be both dirty? >>> even they have the same HVA due to overlaping virtual address space i= n >>> two processes, they still correspond to two physical pages. >>> don't get what's your meaning :) >> >> The point is not leave any corner case that is hard to debug or fix in >> the future. >> >> Let's just start by a single process, the API allows userspace to maps >> HVA to both GPA1 and GPA2. Since it knows GPA1 and GPA2 are equivalent= , >> it's ok to sync just through GPA1. That means if you only log GPA2, it >> won't work. >> > I noted KVM itself doesn't consider such situation (one HVA is mapped > to multiple GPAs), when doing its dirty page tracking. If you look at > kvm_vcpu_mark_page_dirty, it simply finds the unique memslot which > contains the dirty gfn and then set the dirty bit within that slot. It > doesn't attempt to walk all memslots to find out any other GPA which > may be mapped to the same HVA. > > So there must be some disconnect here. let's hear from Paolo first and > understand the rationale behind such situation. Neither did vhost when IOTLB is disabled. And cc Michael who points out=20 this issue at the beginning. Thanks > > Thanks > Kevin