Re: [Qemu-devel] [PULL v2 04/36] virtio: Introduce started flag to VirtioDevice

[Dr. David Alan Gilbert]

* Greg Kurz (address@hidden) wrote:
> On Tue, 28 May 2019 10:08:54 +1000
> David Gibson <address@hidden> wrote:
> 
> > On Fri, May 24, 2019 at 12:19:09PM +0200, Greg Kurz wrote:
> > > On Mon, 20 May 2019 19:10:35 -0400
> > > "Michael S. Tsirkin" <address@hidden> wrote:
> > >   
> > > > From: Xie Yongji <address@hidden>
> > > > 
> > > > The virtio 1.0 transitional devices support driver uses the device
> > > > before setting the DRIVER_OK status bit. So we introduce a started
> > > > flag to indicate whether driver has started the device or not.
> > > > 
> > > > Signed-off-by: Xie Yongji <address@hidden>
> > > > Signed-off-by: Zhang Yu <address@hidden>
> > > > Message-Id: <address@hidden>
> > > > Reviewed-by: Michael S. Tsirkin <address@hidden>
> > > > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > > > ---
> > > >  include/hw/virtio/virtio.h |  2 ++
> > > >  hw/virtio/virtio.c         | 52 ++++++++++++++++++++++++++++++++++++--
> > > >  2 files changed, 52 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > > > index 7140381e3a..27c0efc3d0 100644
> > > > --- a/include/hw/virtio/virtio.h
> > > > +++ b/include/hw/virtio/virtio.h
> > > > @@ -105,6 +105,8 @@ struct VirtIODevice
> > > >      uint16_t device_id;
> > > >      bool vm_running;
> > > >      bool broken; /* device in invalid state, needs reset */
> > > > +    bool started;
> > > > +    bool start_on_kick; /* virtio 1.0 transitional devices support 
> > > > that */
> > > >      VMChangeStateEntry *vmstate;
> > > >      char *bus_name;
> > > >      uint8_t device_endian;
> > > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > > > index 28056a7ef7..5d533ac74e 100644
> > > > --- a/hw/virtio/virtio.c
> > > > +++ b/hw/virtio/virtio.c
> > > > @@ -1162,10 +1162,16 @@ int virtio_set_status(VirtIODevice *vdev, 
> > > > uint8_t val)
> > > >              }
> > > >          }
> > > >      }
> > > > +    vdev->started = val & VIRTIO_CONFIG_S_DRIVER_OK;
> > > > +    if (unlikely(vdev->start_on_kick && vdev->started)) {
> > > > +        vdev->start_on_kick = false;
> > > > +    }
> > > > +
> > > >      if (k->set_status) {
> > > >          k->set_status(vdev, val);
> > > >      }
> > > >      vdev->status = val;
> > > > +
> > > >      return 0;
> > > >  }
> > > >  
> > > > @@ -1208,6 +1214,9 @@ void virtio_reset(void *opaque)
> > > >          k->reset(vdev);
> > > >      }
> > > >  
> > > > +    vdev->start_on_kick = (virtio_host_has_feature(vdev, 
> > > > VIRTIO_F_VERSION_1) &&
> > > > +                          !virtio_vdev_has_feature(vdev, 
> > > > VIRTIO_F_VERSION_1));
> > > > +    vdev->started = false;
> > > >      vdev->broken = false;
> > > >      vdev->guest_features = 0;
> > > >      vdev->queue_sel = 0;
> > > > @@ -1518,14 +1527,21 @@ void virtio_queue_set_align(VirtIODevice *vdev, 
> > > > int n, int align)
> > > >  
> > > >  static bool virtio_queue_notify_aio_vq(VirtQueue *vq)
> > > >  {
> > > > +    bool ret = false;
> > > > +
> > > >      if (vq->vring.desc && vq->handle_aio_output) {
> > > >          VirtIODevice *vdev = vq->vdev;
> > > >  
> > > >          trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
> > > > -        return vq->handle_aio_output(vdev, vq);
> > > > +        ret = vq->handle_aio_output(vdev, vq);
> > > > +
> > > > +        if (unlikely(vdev->start_on_kick)) {
> > > > +            vdev->started = true;
> > > > +            vdev->start_on_kick = false;
> > > > +        }
> > > >      }
> > > >  
> > > > -    return false;
> > > > +    return ret;
> > > >  }
> > > >  
> > > >  static void virtio_queue_notify_vq(VirtQueue *vq)
> > > > @@ -1539,6 +1555,11 @@ static void virtio_queue_notify_vq(VirtQueue *vq)
> > > >  
> > > >          trace_virtio_queue_notify(vdev, vq - vdev->vq, vq);
> > > >          vq->handle_output(vdev, vq);
> > > > +
> > > > +        if (unlikely(vdev->start_on_kick)) {
> > > > +            vdev->started = true;
> > > > +            vdev->start_on_kick = false;
> > > > +        }
> > > >      }
> > > >  }
> > > >  
> > > > @@ -1556,6 +1577,11 @@ void virtio_queue_notify(VirtIODevice *vdev, int 
> > > > n)
> > > >      } else if (vq->handle_output) {
> > > >          vq->handle_output(vdev, vq);
> > > >      }
> > > > +
> > > > +    if (unlikely(vdev->start_on_kick)) {
> > > > +        vdev->started = true;
> > > > +        vdev->start_on_kick = false;
> > > > +    }
> > > >  }
> > > >  
> > > >  uint16_t virtio_queue_vector(VirtIODevice *vdev, int n)
> > > > @@ -1770,6 +1796,13 @@ static bool virtio_broken_needed(void *opaque)
> > > >      return vdev->broken;
> > > >  }
> > > >  
> > > > +static bool virtio_started_needed(void *opaque)
> > > > +{
> > > > +    VirtIODevice *vdev = opaque;
> > > > +
> > > > +    return vdev->started;  
> > > 
> > > Existing machine types don't know about the "virtio/started" subsection. 
> > > This
> > > breaks migration to older QEMUs if the driver has started the device, ie. 
> > > most
> > > probably always when it comes to live migration.
> > > 
> > > My understanding is that we do try to support backward migration though. 
> > > It
> > > is a regular practice in datacenters to migrate workloads without having 
> > > to
> > > take care of the QEMU version. FWIW I had to fix similar issues downstream
> > > many times in the past because customers had filed bugs.
> > > 
> > > Cc'ing David for his opinion.  
> > 
> > Uh.. did you mean to CC me, or Dave Gilbert?
> > 
> 
> Oops... Dave Gilbert indeed, but you're thoughts on that matter are valuable
> as well. I remember being involved in backward migration fixes for spapr
> several times.
> 
> > I mean, I think you're right that we should try to maintain backwards
> > migration, but this isn't really my area of authority.
> > 
> 
> Cc'ing Dave Gilbert :)

Right, I need to maintain backwards migration compatibility; tie the
feature to a machine type so it's only used on newer machine types and
then we'll be safe.

Having said that, what's the symptom when this goes wrong?

Dave

> Cheers,
> 
> --
> Greg


--
Dr. David Alan Gilbert / address@hidden / Manchester, UK