Re: [Qemu-devel] [PATCH RFC 05/11] virtio: introduce legacy virtio devices

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Thu, 30 Oct 2014 23:29:50 +0100
Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:

> On Thu, 30 Oct 2014 19:02:01 +0100
> Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> 
> > On Tue, 28 Oct 2014 16:40:18 +0100
> > Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
> > 
> > > On Tue,  7 Oct 2014 16:40:01 +0200
> > > 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 7aaa953..e6ae3a0 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;
> > > >  }
> > > > 
> > > 
> > > Shouldn't we have some code doing the following somewhere ?
> > > 
> > > if (!virtio_device_is_legacy(vdev)) {
> > >     vdev->device_endian = VIRTIO_DEVICE_ENDIAN_LITTLE;
> > > }
> > > 
> > > also, since virtio-1 is LE only, do we expect device_endian to
> > > be different from VIRTIO_DEVICE_ENDIAN_LITTLE ?
> > 
> > device_endian should not depend on whether the device is legacy or not.
> > virtio_is_big_endian always returns false for virtio-1 devices, though.
> 
> Sorry, I had missed the virtio_is_big_endian() change: it that makes
> device_endian a legacy virtio only matter. 
> So why would we care to migrate the endian subsection when we have a
> virtio-1 device ?  Shouldn't virtio_device_endian_needed() return false
> for virtio-1 ?

Indeeed, we can just leave device_endian at the default value for
virtio-1 devices.