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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 7B557C3A5A6 for ; Thu, 19 Sep 2019 10:11:10 +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 4F02521848 for ; Thu, 19 Sep 2019 10:11:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4F02521848 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]:41818 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iAtP2-0001G6-Cm for qemu-devel@archiver.kernel.org; Thu, 19 Sep 2019 06:11:08 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:49804) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iAtLC-0005IF-GC for qemu-devel@nongnu.org; Thu, 19 Sep 2019 06:07:11 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iAtLB-00088h-3t for qemu-devel@nongnu.org; Thu, 19 Sep 2019 06:07:10 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34328) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iAtLA-00088S-Rv for qemu-devel@nongnu.org; Thu, 19 Sep 2019 06:07:09 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 31D343084037; Thu, 19 Sep 2019 10:07:08 +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 7382119C6A; Thu, 19 Sep 2019 10:07:00 +0000 (UTC) To: Yan Zhao References: <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> <20190919062954.GC18391@joy-OptiPlex-7040> From: Jason Wang Message-ID: <297143e5-a0b5-4873-77bf-7d2a97ec3c7c@redhat.com> Date: Thu, 19 Sep 2019 18:06:52 +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: <20190919062954.GC18391@joy-OptiPlex-7040> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Thu, 19 Sep 2019 10:07:08 +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: "Tian, Kevin" , 'Alex Williamson' , Peter Xu , "qemu-devel@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2019/9/19 =E4=B8=8B=E5=8D=882:29, Yan Zhao wrote: > On Thu, Sep 19, 2019 at 02:32:03PM +0800, Jason Wang wrote: >> 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 HV= A >>>>>>>> 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 checkedhttps://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 insta= nce >>>>> and manipulating on memory slots, adding an extra GPA along side cu= rrent >>>>> IOVA & HVA to ioctl VFIO_IOMMU_MAP_DMA can still let driver knows t= he >>>>> right IOVA->GPA mapping, right? >>>> It looks fragile. Consider HVA was mapped to both GPA1 and GPA2. Gue= st >>>> maps IOVA to GPA2, so we have IOVA GPA2 HVA in the new ioctl and the= n >>>> 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. >> > In that case, cannot log dirty according to HPA. > because kvm cannot tell whether it's an valid case (the two GPAs are eq= uivalent) > or an invalid case (the two GPAs are not equivalent, but with the same > HVA value). > > Right? There no need any examination on whether it was 'valid' or not. It's as=20 simple as logging both GPA1 and GPA2. Then you won't need to care any=20 corner case. Thanks > > Thanks > Yan > >