Re: [Qemu-devel] [PATCH RFC v3 05/12] virtio: introduce legacy virtio devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 26, 2014 at 07:46:38PM +0100, Greg Kurz wrote:
> On Wed, 26 Nov 2014 18:28:36 +0100
> Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> 
> > Introduce a helper function to indicate  whether a virtio device is
> > operating in legacy or virtio standard mode.
> > 
> > It may be used to make decisions about the endianess of virtio accesses
> > and other virtio-1 specific changes, enabling us to support transitional
> > devices.
> > 
> > Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
> > Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> > ---
> >  hw/virtio/virtio.c                |    6 +++++-
> >  include/hw/virtio/virtio-access.h |    4 ++++
> >  include/hw/virtio/virtio.h        |   13 +++++++++++--
> >  3 files changed, 20 insertions(+), 3 deletions(-)
> > 
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 2eb5d3c..4149f45 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -883,7 +883,11 @@ static bool virtio_device_endian_needed(void *opaque)
> >      VirtIODevice *vdev = opaque;
> > 
> >      assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
> > -    return vdev->device_endian != virtio_default_endian();
> > +    if (virtio_device_is_legacy(vdev)) {
> > +        return vdev->device_endian != virtio_default_endian();
> > +    }
> > +    /* Devices conforming to VIRTIO 1.0 or later are always LE. */
> > +    return vdev->device_endian != VIRTIO_DEVICE_ENDIAN_LITTLE;
> >  }
> > 
> 
> Sorry but I still don't understand why we need to stream the device_endian
> subsection if we have a virtio-1 device... this field is only used on
> legacy device paths. Can you share an example where it is needed ?

I think it's needed.
A transitional device can be used with legacy native endian and
modern little endian format.


> >  static const VMStateDescription vmstate_virtio_device_endian = {
> > diff --git a/include/hw/virtio/virtio-access.h b/include/hw/virtio/virtio-access.h
> > index 46456fd..c123ee0 100644
> > --- a/include/hw/virtio/virtio-access.h
> > +++ b/include/hw/virtio/virtio-access.h
> > @@ -19,6 +19,10 @@
> > 
> >  static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
> >  {
> > +    if (!virtio_device_is_legacy(vdev)) {
> > +        /* Devices conforming to VIRTIO 1.0 or later are always LE. */
> > +        return false;
> > +    }
> >  #if defined(TARGET_IS_BIENDIAN)
> >      return virtio_is_big_endian(vdev);
> >  #elif defined(TARGET_WORDS_BIGENDIAN)
> > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > index b408166..40e567c 100644
> > --- a/include/hw/virtio/virtio.h
> > +++ b/include/hw/virtio/virtio.h
> > @@ -275,9 +275,18 @@ void virtio_queue_set_host_notifier_fd_handler(VirtQueue *vq, bool assign,
> >  void virtio_queue_notify_vq(VirtQueue *vq);
> >  void virtio_irq(VirtQueue *vq);
> > 
> > +static inline bool virtio_device_is_legacy(VirtIODevice *vdev)
> > +{
> > +    return !(vdev->guest_features[1] & (1 << (VIRTIO_F_VERSION_1 - 32)));
> > +}
> > +
> >  static inline bool virtio_is_big_endian(VirtIODevice *vdev)
> >  {
> > -    assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
> > -    return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
> > +    if (virtio_device_is_legacy(vdev)) {
> > +        assert(vdev->device_endian != VIRTIO_DEVICE_ENDIAN_UNKNOWN);
> > +        return vdev->device_endian == VIRTIO_DEVICE_ENDIAN_BIG;
> > +    }
> > +    /* Devices conforming to VIRTIO 1.0 or later are always LE. */
> > +    return false;
> >  }
> >  #endif