Linux Virtualization - Oct 2014 - [Qemu-devel] [PATCH RFC 05/11] virtio: introduce legacy virtio devices

On Tue,  7 Oct 2014 16:40:01 +0200
Cornelia Huck <cornelia.huck at 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 at linux.vnet.ibm.com>
> Signed-off-by: Cornelia Huck <cornelia.huck at 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 ?


Cheers.

--
Greg
>  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

On Tue, 28 Oct 2014 16:40:18 +0100
Greg Kurz <gkurz at linux.vnet.ibm.com> wrote:
> On Tue,  7 Oct 2014 16:40:01 +0200
> Cornelia Huck <cornelia.huck at 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 at linux.vnet.ibm.com>
> > Signed-off-by: Cornelia Huck <cornelia.huck at 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.

On Thu, 30 Oct 2014 19:02:01 +0100
Cornelia Huck <cornelia.huck at de.ibm.com> wrote:
> On Tue, 28 Oct 2014 16:40:18 +0100
> Greg Kurz <gkurz at linux.vnet.ibm.com> wrote:
> 
> > On Tue,  7 Oct 2014 16:40:01 +0200
> > Cornelia Huck <cornelia.huck at 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 at linux.vnet.ibm.com>
> > > Signed-off-by: Cornelia Huck <cornelia.huck at 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 ?

--
Greg