From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-comment-return-1303-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: References: <158994007522.397236.7536927155973222190.stgit@ahduyck-workstation> <158994014779.397236.836253101843237605.stgit@ahduyck-workstation> <60278259-9a04-77ab-f96f-c13bc2ffb070@redhat.com> <53e66faa-ed7f-f7c1-6284-1e2a6e63c936@redhat.com> <733c9230-1bec-11f2-9fc9-ed53beedbc66@redhat.com> <20200526173835.4a839ec6.cohuck@redhat.com> <5ECE053E.1000206@intel.com> From: David Hildenbrand Message-ID: <8e33db19-cd4f-6c3d-feff-ecd4437cfcba@redhat.com> Date: Wed, 27 May 2020 14:08:30 +0200 MIME-Version: 1.0 In-Reply-To: <5ECE053E.1000206@intel.com> Content-Language: en-US Subject: [virtio-comment] Re: [virtio-dev] Re: [virtio-comment] [PATCH v3 2/3] content: Document balloon feature page poison Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: quoted-printable To: Wei Wang , Cornelia Huck Cc: Alexander Duyck , "Michael S. Tsirkin" , virtio-comment@lists.oasis-open.org, virtio-dev@lists.oasis-open.org List-ID: On 27.05.20 08:14, Wei Wang wrote: > On 05/26/2020 11:38 PM, Cornelia Huck wrote: >> On Tue, 26 May 2020 17:28:00 +0200 David Hildenbrand >> wrote: >>=20 >>> On 26.05.20 16:50, Alexander Duyck wrote: >>>> On Tue, May 26, 2020 at 1:24 AM David Hildenbrand >>>> wrote: >>>>> Still wondering what to do with free page hinting ... in the >>>>> meantime I'll have a look at free page reporting :) >>>> The problem is it is already out there so I worry we wouldn't >>>> ever be able to get rid of it. At most we could deprecate it, >>>> but we are still stuck with it consuming bit resources and >>>> such. >>> Yeah, that's not an issue, they will simply turn to dead bits >>> with minimal documentation. I just don't see us fixing/supporting >>> that feature, really. Let's see what @MST things when he has time >>> to look into this. >>>=20 >> If free page hinting is broken enough that we don't want anybody to >> try implementing it, we maybe could: >=20 > May I know the issues that you got with FREE_PAGE_HINT? Did you follow the discussion on the spec updates proposed by Alexander? We might have identified a couple of issues in the QEMU side trying to document the semantics of free page hinting. For example: 1. When migration fails in the live stage, before stopping the VM, the guest will not receive a VIRTIO_BALLOON_CMD_ID_DONE. 2. The semantics about what could happen to hinted pages are unclear (and it is unclear if the current QEMU behavior is a BUG or expected). While writing to a hinted page will result in the page to get migrated and not change the value, the guest might suddenly observe a change in the value when only reading the page. Imagine (just as an example) something in a guest like /* page was previously hinted and is now getting reused by the guest */ if (!page_filled_with(page, X)) { =09fill_page_with(page, X); } /* migration finished, value of page changed */ And Alexander pointed out, that the change the guest might observe might not be the change to a zero page. Semantics unclear. There seems to be more related to the async iothread/reset handling + the other fixes I just recently sent. It would be good if you could have a look at the matter. --=20 Thanks, David / dhildenb This publicly archived list offers a means to provide input to the OASIS Virtual I/O Device (VIRTIO) TC. In order to verify user consent to the Feedback License terms and to minimize spam in the list archive, subscription is required before posting. Subscribe: virtio-comment-subscribe@lists.oasis-open.org Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org List help: virtio-comment-help@lists.oasis-open.org List archive: https://lists.oasis-open.org/archives/virtio-comment/ Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lis= ts Committee: https://www.oasis-open.org/committees/virtio/ Join OASIS: https://www.oasis-open.org/join/