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

[Yongji Xie]

On Fri, 24 May 2019 at 18:20, Greg Kurz <address@hidden> 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.
>

If we do need to support backward migration, for this patch, what I
can think of is to only migrate the flag in the case that guest kicks
but not set DRIVER_OK. This could fix backward migration in most case.
Not sure if there is a more general approach...

Thanks,
Yongji