Re: [PATCH V4 5/6] virtio: introduce a mdev based transport

[Jason Wang]

On 2019/10/21 äå5:36, Cornelia Huck wrote:
> On Mon, 21 Oct 2019 13:59:23 +0800
> Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> > On 2019/10/18 äå10:20, Cornelia Huck wrote:
> > > On Thu, 17 Oct 2019 18:48:35 +0800
> > > Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > >   
> > > > This patch introduces a new mdev transport for virtio. This is used to
> > > > use kernel virtio driver to drive the mediated device that is capable
> > > > of populating virtqueue directly.
> > > >
> > > > A new virtio-mdev driver will be registered to the mdev bus, when a
> > > > new virtio-mdev device is probed, it will register the device with
> > > > mdev based config ops. This means it is a software transport between
> > > > mdev driver and mdev device. The transport was implemented through
> > > > device specific ops which is a part of mdev_parent_ops now.
> > > >
> > > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > > > ---
> > > >    drivers/virtio/Kconfig       |   7 +
> > > >    drivers/virtio/Makefile      |   1 +
> > > >    drivers/virtio/virtio_mdev.c | 409 +++++++++++++++++++++++++++++++++++
> > > >    3 files changed, 417 insertions(+)
> > > (...)
> > >   
> > > > +static int virtio_mdev_probe(struct device *dev)
> > > > +{
> > > > +	struct mdev_device *mdev = mdev_from_dev(dev);
> > > > +	const struct virtio_mdev_device_ops *ops = mdev_get_dev_ops(mdev);
> > > > +	struct virtio_mdev_device *vm_dev;
> > > > +	int rc;
> > > > +
> > > > +	vm_dev = devm_kzalloc(dev, sizeof(*vm_dev), GFP_KERNEL);
> > > > +	if (!vm_dev)
> > > > +		return -ENOMEM;
> > > > +
> > > > +	vm_dev->vdev.dev.parent = dev;
> > > > +	vm_dev->vdev.dev.release = virtio_mdev_release_dev;
> > > > +	vm_dev->vdev.config = &virtio_mdev_config_ops;
> > > > +	vm_dev->mdev = mdev;
> > > > +	INIT_LIST_HEAD(&vm_dev->virtqueues);
> > > > +	spin_lock_init(&vm_dev->lock);
> > > > +
> > > > +	vm_dev->version = ops->get_mdev_features(mdev);
> > > > +	if (vm_dev->version != VIRTIO_MDEV_F_VERSION_1) {
> > > > +		dev_err(dev, "VIRTIO_MDEV_F_VERSION_1 is mandatory\n");
> > > > +		return -ENXIO;
> > > > +	}
> > > Hm, so how is that mdev features interface supposed to work? If
> > > VIRTIO_MDEV_F_VERSION_1 is a bit, I would expect this code to test for
> > > its presence, and not for identity.
> >
> > This should be used by driver to detect the which sets of functions and
> > their semantics that could be provided by the device. E.g when driver
> > support both version 2 and version 1 but device only support version 1,
> > driver can switch to use version 1. Btw, Is there a easy way for to test
> > its presence or do you mean doing sanity testing on existence of the
> > mandatory ops that provided by the device?
> What I meant was something like:
>
> features = ops->get_mdev_features(mdev);
> if (features & VIRTIO_MDEV_F_VERSION_1)
> 	vm_dev->version = 1;
> else
> 	//moan about missing support for version 1
>
> Can there be class id specific extra features, or is this only for core
> features? If the latter, maybe also do something like
>
> supported_features = ORED_LIST_OF_FEATURES;
> if (features & ~supported_features)
> 	//moan about extra feature bits


Consider driver can claim to support a list of ids, so I this it's former.

Will do as what you proposed.

Thanks


> > > What will happen if we come up with a version 2? If this is backwards
> > > compatible, will both version 2 and version 1 be set?
> >
> > Yes, I think so, and version 2 should be considered as some extensions
> > of version 1. If it's completely, it should use a new class id.
> Ok, that makes sense.