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

From: Cornelia Huck <[email-masked]>
Date: Mon, 21 Oct 2019 11:36:07 +0200

On Mon, 21 Oct 2019 13:59:23 +0800
Jason Wang <[email-masked]> wrote:

> On 2019/10/18 下午10:20, Cornelia Huck wrote:
> > On Thu, 17 Oct 2019 18:48:35 +0800
> > Jason Wang <[email-masked]> 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 <[email-masked]>
> >> ---
> >>   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

> 
> 
> >
> > 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.