Messages in this thread | | | Date | Wed, 24 Jan 2018 19:28:46 +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 01/24/2018 12:29 PM, Michael S. Tsirkin wrote: > On Mon, Jan 22, 2018 at 07:25:45PM +0800, 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. >> >> >> Best, >> Wei > I think we should just fix races in the driver rather than introduce > random restrictions in the device. > > If device wants to start a new sequence, it should be able to > do just that without a complicated back and forth with several > roundtrips through the driver. >
OK, I've fixed it in the new version, v24. Please have a check there. Thanks. (Other changes based on the comments on v23 have also been included)
Best, Wei
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