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Subject: Re: [virtio-dev] Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

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;

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.


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