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

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Message-ID: <20191018162007.31631039.cohuck@redhat.com>
Date:   Fri, 18 Oct 2019 16:20:07 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
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        alex.williamson@...hat.com, mst@...hat.com, tiwei.bie@...el.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
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        stefanha@...hat.com
Subject: Re: [PATCH V4 5/6] virtio: introduce a mdev based transport

On Thu, 17 Oct 2019 18:48:35 +0800
Jason Wang <jasowang@...hat.com> 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@...hat.com>
> ---
>  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.

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?

> +
> +	vm_dev->vdev.id.device = ops->get_device_id(mdev);
> +	if (vm_dev->vdev.id.device == 0)
> +		return -ENODEV;
> +
> +	vm_dev->vdev.id.vendor = ops->get_vendor_id(mdev);
> +	rc = register_virtio_device(&vm_dev->vdev);
> +	if (rc)
> +		put_device(dev);
> +	else
> +		dev_set_drvdata(dev, vm_dev);
> +
> +	return rc;
> +}

(...)