On Fri, 9 Sep 2016 12:14:31 +0200
Maxime Coquelin <maxime.coque...@redhat.com> wrote:
> This patch adds virtio_test_backend_feature() function to
> enable checking a backend feature before the negociation
> takes place.
>
> It may be used, for example, to check whether the backend
> supports VIRTIO_F_VERSION_1 before enabling modern
> capabilities.
>
> Cc: Marcel Apfelbaum <mar...@redhat.com>
> Cc: Michael S. Tsirkin <m...@redhat.com>
> Cc: qemu-sta...@nongnu.org
> Signed-off-by: Maxime Coquelin <maxime.coque...@redhat.com>
> ---
> hw/virtio/virtio.c | 14 ++++++++++++++
> include/hw/virtio/virtio.h | 2 ++
> 2 files changed, 16 insertions(+)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 74c085c..7ab91a1 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -1481,6 +1481,20 @@ void virtio_vmstate_save(QEMUFile *f, void *opaque,
> size_t size)
> virtio_save(VIRTIO_DEVICE(opaque), f);
> }
>
> +bool virtio_test_backend_feature(VirtIODevice *vdev,
> + unsigned int fbit, Error **errp)
> +{
> + VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
> + uint64_t feature;
> +
> + virtio_add_feature(&feature, fbit);
> +
> + assert(k->get_features != NULL);
> + feature = k->get_features(vdev, feature, errp);
> +
> + return virtio_has_feature(feature, fbit);
> +}
> +
> static int virtio_set_features_nocheck(VirtIODevice *vdev, uint64_t val)
> {
> VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
What happens if you want to test for features that depend upon each
other? The backend may support your feature, but it may withdraw the
feature bit if a dependency is not fullfilled.
You'll probably want to run validation on the whole feature set; but
that is hard if you're too early in the setup process.
