From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52700) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fk8co-00058s-UC for qemu-devel@nongnu.org; Mon, 30 Jul 2018 09:54:15 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fk8cl-0004w4-RT for qemu-devel@nongnu.org; Mon, 30 Jul 2018 09:54:15 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:58168 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fk8cl-0004v3-Ku for qemu-devel@nongnu.org; Mon, 30 Jul 2018 09:54:11 -0400 References: <20180717222721.14019.27548.stgit@gimli.home> <20180730163123-mutt-send-email-mst@kernel.org> From: David Hildenbrand Message-ID: <57aca548-b6f3-eae3-3b2e-a25523294a7d@redhat.com> Date: Mon, 30 Jul 2018 15:54:04 +0200 MIME-Version: 1.0 In-Reply-To: <20180730163123-mutt-send-email-mst@kernel.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC PATCH 0/3] Balloon inhibit enhancements List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" , Alex Williamson Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org On 30.07.2018 15:34, Michael S. Tsirkin wrote: > On Tue, Jul 17, 2018 at 04:47:31PM -0600, Alex Williamson wrote: >> Directly assigned vfio devices have never been compatible with >> ballooning. Zapping MADV_DONTNEED pages happens completely >> independent of vfio page pinning and IOMMU mapping, leaving us with >> inconsistent GPA to HPA mapping between vCPUs and assigned devices >> when the balloon deflates. Mediated devices can theoretically do >> better, if we make the assumption that the mdev vendor driver is fully >> synchronized to the actual working set of the guest driver. In that >> case the guest balloon driver should never be able to allocate an mdev >> pinned page for balloon inflation. Unfortunately, QEMU can't know the >> workings of the vendor driver pinning, and doesn't actually know the >> difference between mdev devices and directly assigned devices. Until >> we can sort out how the vfio IOMMU backend can tell us if ballooning >> is safe, the best approach is to disabling ballooning any time a vfio >> devices is attached. >> >> To do that, simply make the balloon inhibitor a counter rather than a >> boolean, fixup a case where KVM can then simply use the inhibit >> interface, and inhibit ballooning any time a vfio device is attached. >> I'm expecting we'll expose some sort of flag similar to >> KVM_CAP_SYNC_MMU from the vfio IOMMU for cases where we can resolve >> this. An addition we could consider here would be yet another device >> option for vfio, such as x-disable-balloon-inhibit, in case there are >> mdev devices that behave in a manner compatible with ballooning. >> >> Please let me know if this looks like a good idea. Thanks, >> >> Alex > > It's probably the only a reasonable thing to do for this release. > > Long term however, why can't balloon notify vfio as pages are > added and removed? VFIO could update its mappings then. What if the guest is rebooted and pages are silently getting reused without getting a deflation request first? -- Thanks, David / dhildenb