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bh=JhwaB6r9gICIWyYPkf3yBo2ARG9VZb6Y0ndCYozc44E=; b=TD2xbGxrXsJIFAF8+/W92RpUTdiFEXfLyYdnM0n5g1p6uT0Bg8O0/P6U+Q/a+JLwDnydGd uFXG0rv8Ysw3Od8ys+WdH0SnbpeUyEV/ZAWj80UU5+WLVFwA7aUsuun8MX8WL6lFOLjayp QlHNhugdjfTFQavvLhN/BScK6xQ9VnQ= 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-523-LSLY1spFPZqdxCheUL2OZg-1; Tue, 29 Sep 2020 13:06:13 -0400 X-MC-Unique: LSLY1spFPZqdxCheUL2OZg-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id AF1491054F8E; Tue, 29 Sep 2020 17:06:12 +0000 (UTC) Received: from [10.36.112.157] (ovpn-112-157.ams2.redhat.com [10.36.112.157]) by smtp.corp.redhat.com (Postfix) with ESMTP id 08CDD5C1C4; Tue, 29 Sep 2020 17:05:54 +0000 (UTC) Subject: Re: [PATCH PROTOTYPE 0/6] virtio-mem: vfio support To: "Dr. David Alan Gilbert" References: <20200924160423.106747-1-david@redhat.com> <20200929170238.GN2826@work-vm> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20200929170238.GN2826@work-vm> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 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-Type: text/plain; charset=utf-8 Content-Language: en-US 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-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/29 02:22:44 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.687, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=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: Pankaj Gupta , "Michael S. Tsirkin" , qemu-devel@nongnu.org, Peter Xu , Luiz Capitulino , Auger Eric , Alex Williamson , Wei Yang , Paolo Bonzini , Igor Mammedov Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 29.09.20 19:02, Dr. David Alan Gilbert wrote: > * David Hildenbrand (david@redhat.com) wrote: >> This is a quick and dirty (1.5 days of hacking) prototype to make >> vfio and virtio-mem play together. The basic idea was the result of Alex >> brainstorming with me on how to tackle this. >> >> A virtio-mem device manages a memory region in guest physical address >> space, represented as a single (currently large) memory region in QEMU. >> Before the guest is allowed to use memory blocks, it must coordinate with >> the hypervisor (plug blocks). After a reboot, all memory is usually >> unplugged - when the guest comes up, it detects the virtio-mem device and >> selects memory blocks to plug (based on requests from the hypervisor). >> >> Memory hot(un)plug consists of (un)plugging memory blocks via a virtio-mem >> device (triggered by the guest). When unplugging blocks, we discard the >> memory. In contrast to memory ballooning, we always know which memory >> blocks a guest may use - especially during a reboot, after a crash, or >> after kexec. >> >> The issue with vfio is, that it cannot deal with random discards - for this >> reason, virtio-mem and vfio can currently only run mutually exclusive. >> Especially, vfio would currently map the whole memory region (with possible >> only little/no plugged blocks), resulting in all pages getting pinned and >> therefore resulting in a higher memory consumption than expected (turning >> virtio-mem basically useless in these environments). >> >> To make vfio work nicely with virtio-mem, we have to map only the plugged >> blocks, and map/unmap properly when plugging/unplugging blocks (including >> discarding of RAM when unplugging). We achieve that by using a new notifier >> mechanism that communicates changes. >> >> It's important to map memory in the granularity in which we could see >> unmaps again (-> virtio-mem block size) - so when e.g., plugging >> consecutive 100 MB with a block size of 2MB, we need 50 mappings. When >> unmapping, we can use a single vfio_unmap call for the applicable range. >> We expect that the block size of virtio-mem devices will be fairly large >> in the future (to not run out of mappings and to improve hot(un)plug >> performance), configured by the user, when used with vfio (e.g., 128MB, >> 1G, ...) - Linux guests will still have to be optimized for that. > > This seems pretty painful for those few TB mappings. > Also the calls seem pretty painful; maybe it'll be possible to have > calls that are optimised for making multiple consecutive mappings. Exactly the future I imagine. This patchset is with no kernel interface additions - once we have an optimized interface that understands consecutive mappings (and the granularity), we can use that instead. The prototype already prepared for that by notifying about consecutive ranges. Thanks! -- Thanks, David / dhildenb