From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Hildenbrand Subject: Re: [RFC PATCH 0/3] Balloon inhibit enhancements Date: Mon, 30 Jul 2018 15:54:04 +0200 Message-ID: <57aca548-b6f3-eae3-3b2e-a25523294a7d@redhat.com> References: <20180717222721.14019.27548.stgit@gimli.home> <20180730163123-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org To: "Michael S. Tsirkin" , Alex Williamson Return-path: In-Reply-To: <20180730163123-mutt-send-email-mst@kernel.org> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel2=m.gmane.org@nongnu.org Sender: "Qemu-devel" List-Id: 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