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

From: Jason Wang <jasowang@redhat.com>
To: Cornelia Huck <cohuck@redhat.com>
Cc: kvm@vger.kernel.org, linux-s390@vger.kernel.org,
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Subject: Re: [PATCH V4 5/6] virtio: introduce a mdev based transport
Date: Mon, 21 Oct 2019 18:13:02 +0800	[thread overview]
Message-ID: <1aa59fea-cae5-6303-4a94-51493d5748ba@redhat.com> (raw)
In-Reply-To: <20191021113607.16b26d9d.cohuck@redhat.com>

On 2019/10/21 下午5:36, Cornelia Huck wrote:
> On Mon, 21 Oct 2019 13:59:23 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> On 2019/10/18 下午10:20, Cornelia Huck wrote:
>>> On Thu, 17 Oct 2019 18:48:35 +0800
>>> Jason Wang <jasowang@redhat.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@redhat.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.
>>
>> 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.
>