From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Wang Subject: Re: [virtio-dev] Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ Date: Wed, 24 Jan 2018 11:18:40 +0800 Message-ID: <5A67FB10.2050201@intel.com> References: <1516165812-3995-1-git-send-email-wei.w.wang@intel.com> <1516165812-3995-3-git-send-email-wei.w.wang@intel.com> <20180117180337-mutt-send-email-mst@kernel.org> <5A616995.4050702@intel.com> <20180119143517-mutt-send-email-mst@kernel.org> <5A65CA39.2070906@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <5A65CA39.2070906@intel.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: "Michael S. Tsirkin" Cc: yang.zhang.wz@gmail.com, virtio-dev@lists.oasis-open.org, riel@redhat.com, quan.xu0@gmail.com, kvm@vger.kernel.org, liliang.opensource@gmail.com, linux-kernel@vger.kernel.org, mhocko@kernel.org, linux-mm@kvack.org, pbonzini@redhat.com, akpm@linux-foundation.org, nilal@redhat.com, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org On 01/22/2018 07:25 PM, Wei Wang wrote: > On 01/19/2018 08:39 PM, Michael S. Tsirkin wrote: >> On Fri, Jan 19, 2018 at 11:44:21AM +0800, Wei Wang wrote: >>> On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote: >>>> On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote: >>>> >>>>> + vb->start_cmd_id = cmd_id; >>>>> + queue_work(vb->balloon_wq, &vb->report_free_page_work); >>>> It seems that if a command was already queued (with a different id), >>>> this will result in new command id being sent to host twice, which >>>> will >>>> likely confuse the host. >>> I think that case won't happen, because >>> - the host sends a cmd id to the guest via the config, while the >>> guest acks >>> back the received cmd id via the virtqueue; >>> - the guest ack back a cmd id only when a new cmd id is received >>> from the >>> host, that is the above check: >>> >>> if (cmd_id != vb->start_cmd_id) { --> the driver only queues the >>> reporting work only when a new cmd id is received >>> /* >>> * Host requests to start the reporting by >>> sending a >>> * new cmd id. >>> */ >>> WRITE_ONCE(vb->report_free_page, true); >>> vb->start_cmd_id = cmd_id; >>> queue_work(vb->balloon_wq, >>> &vb->report_free_page_work); >>> } >>> >>> So the same cmd id wouldn't queue the reporting work twice. >>> >> Like this: >> >> vb->start_cmd_id = cmd_id; >> queue_work(vb->balloon_wq, &vb->report_free_page_work); >> >> command id changes >> >> vb->start_cmd_id = cmd_id; >> >> work executes >> >> queue_work(vb->balloon_wq, &vb->report_free_page_work); >> >> work executes again >> > > If we think about the whole working flow, I think this case couldn't > happen: > > 1) device send cmd_id=1 to driver; > 2) driver receives cmd_id=1 in the config and acks cmd_id=1 to the > device via the vq; > 3) device revives cmd_id=1; > 4) device wants to stop the reporting by sending cmd_id=STOP; > 5) driver receives cmd_id=STOP from the config, and acks cmd_id=STOP > to the device via the vq; > 6) device sends cmd_id=2 to driver; > ... > > cmd_id=2 won't come after cmd_id=1, there will be a STOP cmd in > between them (STOP won't queue the work). > > How about defining the correct device behavior in the spec: > The device Should NOT send a second cmd id to the driver until a STOP > cmd ack for the previous cmd id has been received from the guest. Thanks for the comments, and I adopted most of them in the new posted v23 patches. The above discussion is the one that I haven't included. If you could still see issues in the above analysis, please let me know. Thanks. Best, Wei