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bh=CcvkI9DvDmyyKLWQws3E4fQ6f20REhQ20ObKduBEhxs=; b=VaSZv37c9XZ97CDOlgfdJaUfp80AkW8KgxkLNFYicbjgaTYZzSdBrNuyF8t+8a9HYkVJp/ 9RHNefDCHP0Ul2sFke28HqopH/zN/V0DGWiUe7fBfOtTUrzKQtP1Gs2K+9J3G+XRcZwRBc i2r4iBEhXLkMhdUZQKRErb0dVeRg8y8= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-407-48mmDB5jO4mXx0KoRKrFIg-1; Wed, 04 Nov 2020 07:45:29 -0500 X-MC-Unique: 48mmDB5jO4mXx0KoRKrFIg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 215371868403; Wed, 4 Nov 2020 12:45:28 +0000 (UTC) Received: from x1.home (ovpn-112-213.phx2.redhat.com [10.3.112.213]) by smtp.corp.redhat.com (Postfix) with ESMTP id 7D88F6EF73; Wed, 4 Nov 2020 12:45:27 +0000 (UTC) Date: Wed, 4 Nov 2020 05:45:27 -0700 From: Alex Williamson To: Kirti Wankhede Subject: Re: [PATCH v1] docs/devel: Add VFIO device migration documentation Message-ID: <20201104054527.22bbace7@x1.home> In-Reply-To: References: <1603950791-27236-1-git-send-email-kwankhede@nvidia.com> <20201029125221.69352b48.cohuck@redhat.com> <9479dffd-e434-e336-6ed8-07fc2edd2453@nvidia.com> <20201029130519.7eb1e704@w520.home> <47f8ccea-f75a-dfb7-b646-28d5123b322f@nvidia.com> <20201103132758.04b18f5c@w520.home> Organization: Red Hat MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=alex.williamson@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=alex.williamson@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/03 00:03:41 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: mcrossley@nvidia.com, cjia@nvidia.com, Cornelia Huck , qemu-devel@nongnu.org, dnigam@nvidia.com, philmd@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, 4 Nov 2020 13:25:40 +0530 Kirti Wankhede wrote: > On 11/4/2020 1:57 AM, Alex Williamson wrote: > > On Wed, 4 Nov 2020 01:18:12 +0530 > > Kirti Wankhede wrote: > > > >> On 10/30/2020 12:35 AM, Alex Williamson wrote: > >>> On Thu, 29 Oct 2020 23:11:16 +0530 > >>> Kirti Wankhede wrote: > >>> > >> > >> > >> > >>>>>> +System memory dirty pages tracking > >>>>>> +---------------------------------- > >>>>>> + > >>>>>> +A ``log_sync`` memory listener callback is added to mark system memory pages > >>>>> > >>>>> s/is added to mark/marks those/ > >>>>> > >>>>>> +as dirty which are used for DMA by VFIO device. Dirty pages bitmap is queried > >>>>> > >>>>> s/by/by the/ > >>>>> s/Dirty/The dirty/ > >>>>> > >>>>>> +per container. All pages pinned by vendor driver through vfio_pin_pages() > >>>>> > >>>>> s/by/by the/ > >>>>> > >>>>>> +external API have to be marked as dirty during migration. When there are CPU > >>>>>> +writes, CPU dirty page tracking can identify dirtied pages, but any page pinned > >>>>>> +by vendor driver can also be written by device. There is currently no device > >>>>> > >>>>> s/by/by the/ (x2) > >>>>> > >>>>>> +which has hardware support for dirty page tracking. So all pages which are > >>>>>> +pinned by vendor driver are considered as dirty. > >>>>>> +Dirty pages are tracked when device is in stop-and-copy phase because if pages > >>>>>> +are marked dirty during pre-copy phase and content is transfered from source to > >>>>>> +destination, there is no way to know newly dirtied pages from the point they > >>>>>> +were copied earlier until device stops. To avoid repeated copy of same content, > >>>>>> +pinned pages are marked dirty only during stop-and-copy phase. > >>>> > >>>> > >>>>> Let me take a quick stab at rewriting this paragraph (not sure if I > >>>>> understood it correctly): > >>>>> > >>>>> "Dirty pages are tracked when the device is in the stop-and-copy phase. > >>>>> During the pre-copy phase, it is not possible to distinguish a dirty > >>>>> page that has been transferred from the source to the destination from > >>>>> newly dirtied pages, which would lead to repeated copying of the same > >>>>> content. Therefore, pinned pages are only marked dirty during the > >>>>> stop-and-copy phase." ? > >>>>> > >>>> > >>>> I think above rephrase only talks about repeated copying in pre-copy > >>>> phase. Used "copied earlier until device stops" to indicate both > >>>> pre-copy and stop-and-copy till device stops. > >>> > >>> > >>> Now I'm confused, I thought we had abandoned the idea that we can only > >>> report pinned pages during stop-and-copy. Doesn't the device needs to > >>> expose its dirty memory footprint during the iterative phase regardless > >>> of whether that causes repeat copies? If QEMU iterates and sees that > >>> all memory is still dirty, it may have transferred more data, but it > >>> can actually predict if it can achieve its downtime tolerances. Which > >>> is more important, less data transfer or predictability? Thanks, > >>> > >> > >> Even if QEMU copies and transfers content of all sys mem pages during > >> pre-copy (worst case with IOMMU backed mdev device when its vendor > >> driver is not smart to pin pages explicitly and all sys mem pages are > >> marked dirty), then also its prediction about downtime tolerance will > >> not be correct, because during stop-and-copy again all pages need to be > >> copied as device can write to any of those pinned pages. > > > > I think you're only reiterating my point. If QEMU copies all of guest > > memory during the iterative phase and each time it sees that all memory > > is dirty, such as if CPUs or devices (including assigned devices) are > > dirtying pages as fast as it copies them (or continuously marks them > > dirty), then QEMU can predict that downtime will require copying all > > pages. > > But as of now there is no way to know if device has dirtied pages during > iterative phase. This claim doesn't make any sense, pinned pages are considered persistently dirtied, during the iterative phase and while stopped. > > If instead devices don't mark dirty pages until the VM is > > stopped, then QEMU might iterate through memory copy and predict a short > > downtime because not much memory is dirty, only to be surprised that > > all of memory is suddenly dirty. At that point it's too late, the VM > > is already stopped, the predicted short downtime takes far longer than > > expected. This is exactly why we made the kernel interface mark pinned > > pages persistently dirty when it was proposed that we only report > > pinned pages once. Thanks, > > > > Since there is no way to know if device dirtied pages during iterative > phase, QEMU should query pinned pages in stop-and-copy phase. As above, I don't believe this is true. > Whenever there will be hardware support or some software mechanism to > report pages dirtied by device then we will add a capability bit in > migration capability and based on that capability bit qemu/user space > app should decide to query dirty pages in iterative phase. Yes, we could advertise support for fine granularity dirty page tracking, but I completely disagree that we should consider pinned pages clean until suddenly exposing them as dirty once the VM is stopped. Thanks, Alex