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Subject: Re: [PATCH V4 5/6] virtio: introduce a mdev based transport
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     kvm@vger.kernel.org, linux-s390@vger.kernel.org,
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References: <20191017104836.32464-1-jasowang@redhat.com>
 <20191017104836.32464-6-jasowang@redhat.com>
 <20191018162007.31631039.cohuck@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <2bb5645b-5c46-9cae-0571-65c302f51cf2@redhat.com>
Date:   Mon, 21 Oct 2019 13:59:23 +0800
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On 2019/10/18 =E4=B8=8B=E5=8D=8810:20, 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)
>> +{
>> +=09struct mdev_device *mdev =3D mdev_from_dev(dev);
>> +=09const struct virtio_mdev_device_ops *ops =3D mdev_get_dev_ops(mdev);
>> +=09struct virtio_mdev_device *vm_dev;
>> +=09int rc;
>> +
>> +=09vm_dev =3D devm_kzalloc(dev, sizeof(*vm_dev), GFP_KERNEL);
>> +=09if (!vm_dev)
>> +=09=09return -ENOMEM;
>> +
>> +=09vm_dev->vdev.dev.parent =3D dev;
>> +=09vm_dev->vdev.dev.release =3D virtio_mdev_release_dev;
>> +=09vm_dev->vdev.config =3D &virtio_mdev_config_ops;
>> +=09vm_dev->mdev =3D mdev;
>> +=09INIT_LIST_HEAD(&vm_dev->virtqueues);
>> +=09spin_lock_init(&vm_dev->lock);
>> +
>> +=09vm_dev->version =3D ops->get_mdev_features(mdev);
>> +=09if (vm_dev->version !=3D VIRTIO_MDEV_F_VERSION_1) {
>> +=09=09dev_err(dev, "VIRTIO_MDEV_F_VERSION_1 is mandatory\n");
>> +=09=09return -ENXIO;
>> +=09}
> 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=20
their semantics that could be provided by the device. E.g when driver=20
support both version 2 and version 1 but device only support version 1,=20
driver can switch to use version 1. Btw, Is there a easy way for to test=20
its presence or do you mean doing sanity testing on existence of the=20
mandatory ops that provided by the device?


>
> 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=20
of version 1. If it's completely, it should use a new class id.

Thanks


>
>> +
>> +=09vm_dev->vdev.id.device =3D ops->get_device_id(mdev);
>> +=09if (vm_dev->vdev.id.device =3D=3D 0)
>> +=09=09return -ENODEV;
>> +
>> +=09vm_dev->vdev.id.vendor =3D ops->get_vendor_id(mdev);
>> +=09rc =3D register_virtio_device(&vm_dev->vdev);
>> +=09if (rc)
>> +=09=09put_device(dev);
>> +=09else
>> +=09=09dev_set_drvdata(dev, vm_dev);
>> +
>> +=09return rc;
>> +}
> (...)