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

[Wei Wang]

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