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

On Mon, Jan 22, 2018 at 07:25:45PM +0800, Wei Wang wrote:

On 01/19/2018 08:39 PM, Michael S. Tsirkin wrote:

quoted

On Fri, Jan 19, 2018 at 11:44:21AM +0800, Wei Wang wrote:

quoted

On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote:

quoted

On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote:

quoted

+		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.

-- 
MST