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

[Wei Wang <wei.w.wang@intel.com>]

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;
> > >                          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.


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.

Best,
Wei