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

[Greg Kurz]

On Mon, 27 May 2019 21:04:38 +0800
Yongji Xie <address@hidden> wrote:

> On Mon, 27 May 2019 at 18:44, Greg Kurz <address@hidden> wrote:
> >
> > On Fri, 24 May 2019 19:56:06 +0800
> > Yongji Xie <address@hidden> wrote:
> >  
> > > 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.  
> >
> > You mean something like that ?
> >
> > static bool virtio_started_needed(void *opaque)
> > {
> >     VirtIODevice *vdev = opaque;
> >
> >     return vdev->started && !(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK);
> > }
> >  
> 
> Yes.
> 
> > > Not sure if there is a more general approach...
> > >  
> >
> > Another approach would be to only implement the started flag for
> > machine version > 4.0. This can be achieved by adding a "use-started"
> > property to the base virtio device, true by default and set to
> > false by hw_compat_4_0.
> >  
> 
> If so, the problem that I tried to fix in this patch would still be
> exist in old machine type, right?
> 

Yes.

> Now I think the first approach may be more reasonable. Blocking a
> migration that will cause guest hang instead of allowing that.
> 

Fair enough but it may be difficult for the end user to understand
why the migration failed in the first place... What about detecting
the hang condition (kick before DRIVER_OK) in the source, add
a migration blocker with an explicit message and remove the blocker
once DRIVER_OK is set ?

> Thanks,
> Yongji