Messages in this thread | | | Date | Mon, 22 Jan 2018 19:25:45 +0800 | From | Wei Wang <> | Subject | Re: [virtio-dev] Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ |
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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.
Best, Wei
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