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

Michael S. Tsirkin
2014-11-26T18:51:22Z

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