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 998DFC3A5A6 for ; Fri, 20 Sep 2019 01:22:27 +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 70D44214AF for ; Fri, 20 Sep 2019 01:22:27 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 70D44214AF 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]:50464 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iB7cw-0000dw-Jo for qemu-devel@archiver.kernel.org; Thu, 19 Sep 2019 21:22:26 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:49621) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iB7Zt-000594-TV for qemu-devel@nongnu.org; Thu, 19 Sep 2019 21:19:19 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iB7Wa-0003Xk-SV for qemu-devel@nongnu.org; Thu, 19 Sep 2019 21:15:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36428) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iB7Wa-0003XS-KV for qemu-devel@nongnu.org; Thu, 19 Sep 2019 21:15:52 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 71EA318C4270; Fri, 20 Sep 2019 01:15:51 +0000 (UTC) Received: from [10.72.12.88] (ovpn-12-88.pek2.redhat.com [10.72.12.88]) by smtp.corp.redhat.com (Postfix) with ESMTP id 34C685D6B0; Fri, 20 Sep 2019 01:15:41 +0000 (UTC) Subject: Re: [Qemu-devel] vhost, iova, and dirty page tracking To: "Michael S. Tsirkin" References: <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> <7906030a-00e3-012d-da4f-bb0c1b2901b2@redhat.com> <20190919100545-mutt-send-email-mst@kernel.org> From: Jason Wang Message-ID: <1373899c-4cbc-cd23-512b-2994e6b26a27@redhat.com> Date: Fri, 20 Sep 2019 09:15:40 +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: <20190919100545-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.62]); Fri, 20 Sep 2019 01:15:51 +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 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 , "Tian, Kevin" , "Zhao, Yan Y" , 'Alex Williamson' , "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=8810:06, Michael S. Tsirkin wrote: > On Thu, Sep 19, 2019 at 05:37:48PM +0800, Jason Wang wrote: >> 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 neit= her >>>> 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= all >>>>>>>>> 1:1, instead of N:1. From guest p.o.v every writable GPA implie= s 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.= So we >>>>>>>> probably need to take care it. >>>>>>>> >>>>>>> yes, in KVM API level, it does not forbid two slots to have the s= ame >>>> HVA(slot->userspace_addr). >>>>>>> But >>>>>>> (1) there's only one kvm instance for each vm for each qemu proce= ss. >>>>>>> (2) all ramblock->host (corresponds to HVA and slot->userspace_ad= dr) >>>> 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= of >>>> 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 i= s the >>>>>> only user or any its behavior. If you had you should limit it in t= he 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 = will >>>>>> 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= do >>>>>> that is to maintain an independent HVA to GPA mapping like what KV= M >>>> or >>>>>> vhost did. >>>>>> >>>>> why GPA1 and GPA2 should be both dirty? >>>>> even they have the same HVA due to overlaping virtual address space= in >>>>> 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 ma= ps >>>> HVA to both GPA1 and GPA2. Since it knows GPA1 and GPA2 are equivale= nt, >>>> 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. I= t >>> 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 an= d >>> understand the rationale behind such situation. >> >> Neither did vhost when IOTLB is disabled. And cc Michael who points ou= t this >> issue at the beginning. >> >> Thanks >> >> >>> Thanks >>> Kevin > Yes, we fixed with a kind of a work around, at the time I proposed > a new interace to fix it fully. I don't think we ever got around > to implementing it - right? Paolo said userspace just need to sync through all GPAs, so my=20 understanding is that work around is ok by redundant, so did the API you=20 proposed. Anything I miss? Thanks