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[91.12.104.16]) by smtp.gmail.com with ESMTPSA id w15sm8634967wrk.77.2021.11.02.10.10.13 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 02 Nov 2021 10:10:14 -0700 (PDT) Message-ID: Date: Tue, 2 Nov 2021 18:10:13 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v1 00/12] virtio-mem: Expose device memory via multiple memslots To: "Michael S. Tsirkin" References: <20211027124531.57561-1-david@redhat.com> <20211101181352-mutt-send-email-mst@kernel.org> <20211102072843-mutt-send-email-mst@kernel.org> <171c8ed0-d55e-77ef-963b-6d836729ef4b@redhat.com> <20211102111228-mutt-send-email-mst@kernel.org> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20211102111228-mutt-send-email-mst@kernel.org> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -60 X-Spam_score: -6.1 X-Spam_bar: ------ X-Spam_report: (-6.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.702, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-2.549, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , kvm@vger.kernel.org, Richard Henderson , Stefan Hajnoczi , qemu-devel@nongnu.org, Peter Xu , "Dr . David Alan Gilbert" , Sebastien Boeuf , Igor Mammedov , Ani Sinha , Paolo Bonzini , Hui Zhu , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 02.11.21 18:06, Michael S. Tsirkin wrote: > On Tue, Nov 02, 2021 at 12:55:17PM +0100, David Hildenbrand wrote: >> On 02.11.21 12:35, Michael S. Tsirkin wrote: >>> On Tue, Nov 02, 2021 at 09:33:55AM +0100, David Hildenbrand wrote: >>>> On 01.11.21 23:15, Michael S. Tsirkin wrote: >>>>> On Wed, Oct 27, 2021 at 02:45:19PM +0200, David Hildenbrand wrote: >>>>>> This is the follow-up of [1], dropping auto-detection and vhost-user >>>>>> changes from the initial RFC. >>>>>> >>>>>> Based-on: 20211011175346.15499-1-david@redhat.com >>>>>> >>>>>> A virtio-mem device is represented by a single large RAM memory region >>>>>> backed by a single large mmap. >>>>>> >>>>>> Right now, we map that complete memory region into guest physical addres >>>>>> space, resulting in a very large memory mapping, KVM memory slot, ... >>>>>> although only a small amount of memory might actually be exposed to the VM. >>>>>> >>>>>> For example, when starting a VM with a 1 TiB virtio-mem device that only >>>>>> exposes little device memory (e.g., 1 GiB) towards the VM initialliy, >>>>>> in order to hotplug more memory later, we waste a lot of memory on metadata >>>>>> for KVM memory slots (> 2 GiB!) and accompanied bitmaps. Although some >>>>>> optimizations in KVM are being worked on to reduce this metadata overhead >>>>>> on x86-64 in some cases, it remains a problem with nested VMs and there are >>>>>> other reasons why we would want to reduce the total memory slot to a >>>>>> reasonable minimum. >>>>>> >>>>>> We want to: >>>>>> a) Reduce the metadata overhead, including bitmap sizes inside KVM but also >>>>>> inside QEMU KVM code where possible. >>>>>> b) Not always expose all device-memory to the VM, to reduce the attack >>>>>> surface of malicious VMs without using userfaultfd. >>>>> >>>>> I'm confused by the mention of these security considerations, >>>>> and I expect users will be just as confused. >>>> >>>> Malicious VMs wanting to consume more memory than desired is only >>>> relevant when running untrusted VMs in some environments, and it can be >>>> caught differently, for example, by carefully monitoring and limiting >>>> the maximum memory consumption of a VM. We have the same issue already >>>> when using virtio-balloon to logically unplug memory. For me, it's a >>>> secondary concern ( optimizing a is much more important ). >>>> >>>> Some users showed interest in having QEMU disallow access to unplugged >>>> memory, because coming up with a maximum memory consumption for a VM is >>>> hard. This is one step into that direction without having to run with >>>> uffd enabled all of the time. >>> >>> Sorry about missing the memo - is there a lot of overhead associated >>> with uffd then? >> >> When used with huge/gigantic pages, we don't particularly care. >> >> For other memory backends, we'll have to route any population via the >> uffd handler: guest accesses a 4k page -> place a 4k page from user >> space. Instead of the kernel automatically placing a THP, we'd be >> placing single 4k pages and have to hope the kernel will collapse them >> into a THP later. > > How much value there is in a THP given it's not present? If you don't place a THP right during the first page fault inside the THP region, you'll have to rely on khugepagd to eventually place a huge page later -- and manually fault in each and every 4k page. I haven't done any performance measurements so far. Going via userspace on every 4k fault will most certainly hurt performance when first touching memory. -- Thanks, David / dhildenb