Re: [Qemu-devel] [PATCH v2] virtio-rng: process pending requests when driver is ready

[Pankaj Gupta]

> 
> On Wed, Jun 27, 2018 at 04:55:20PM +0530, Pankaj Gupta wrote:
> >  virtio-rng device causing old guest kernels(2.6.32) to hang on latest
> >  qemu.
> >  The driver attempts to read from the virtio-rng device too early in it's
> >  initialization. Qemu detects guest is not ready and returns, resulting in
> >  hang.
> 
> Presumably this all worked previously at some point, so do you know what
> change in QEMU has caused a regression in this regard.

Latest qemu(2.x) has this issue with older guest kernel(2.6.32) versions.
But it does work properly with upstream guest kernel because the way virtio-rng
driver is reworked, it sends request to virtio-rng driver when(before) there is 
read call. 

If guest driver is ready when it notifies qemu this works fine. There are number
of changes in guest virtio/driver code infrastructure as well as qemu side, I 
don't 
know exact commit after which this stopped working for older guest kernels. 
 
Thanks,
Pankaj

> 
> > 
> >  Fix is to handle pending request when guest is running and driver status
> >  is
> >  set to 'VIRTIO_CONFIG_S_DRIVER_OK'.
> > 
> > Reported-by: Sergio lopez <address@hidden>
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
> > Signed-off-by: Pankaj Gupta <address@hidden>
> > ---
> > v1->v2: Update new status value early only for virtio-rng (Stefan)
> > 
> >  hw/virtio/virtio-rng.c | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/hw/virtio/virtio-rng.c b/hw/virtio/virtio-rng.c
> > index 289bbcac03..855f1b41d1 100644
> > --- a/hw/virtio/virtio-rng.c
> > +++ b/hw/virtio/virtio-rng.c
> > @@ -156,6 +156,19 @@ static void check_rate_limit(void *opaque)
> >      vrng->activate_timer = true;
> >  }
> >  
> > +static void virtio_rng_set_status(VirtIODevice *vdev, uint8_t status)
> > +{
> > +    VirtIORNG *vrng = VIRTIO_RNG(vdev);
> > +
> > +    if (!vdev->vm_running) {
> > +        return;
> > +    }
> > +    vdev->status = status;
> > +
> > +    /* Something changed, try to process buffers */
> > +    virtio_rng_process(vrng);
> > +}
> > +
> >  static void virtio_rng_device_realize(DeviceState *dev, Error **errp)
> >  {
> >      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > @@ -261,6 +274,7 @@ static void virtio_rng_class_init(ObjectClass *klass,
> > void *data)
> >      vdc->realize = virtio_rng_device_realize;
> >      vdc->unrealize = virtio_rng_device_unrealize;
> >      vdc->get_features = get_features;
> > +    vdc->set_status = virtio_rng_set_status;
> >  }
> >  
> >  static const TypeInfo virtio_rng_info = {
> > --
> > 2.14.3
> > 
> > 
> 
> Regards,
> Daniel
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