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Subject: Re: [PATCH V4 5/6] virtio: introduce a mdev based transport
To:     Cornelia Huck <cohuck@redhat.com>
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References: <20191017104836.32464-1-jasowang@redhat.com>
 <20191017104836.32464-6-jasowang@redhat.com>
 <20191018162007.31631039.cohuck@redhat.com>
 <2bb5645b-5c46-9cae-0571-65c302f51cf2@redhat.com>
 <20191021113607.16b26d9d.cohuck@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <1aa59fea-cae5-6303-4a94-51493d5748ba@redhat.com>
Date:   Mon, 21 Oct 2019 18:13:02 +0800
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On 2019/10/21 =E4=B8=8B=E5=8D=885:36, Cornelia Huck wrote:
> On Mon, 21 Oct 2019 13:59:23 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> 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:
>>>  =20
>>>> 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(+)
>>> (...)
>>>  =20
>>>> +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
>> 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 =3D ops->get_mdev_features(mdev);
> if (features & VIRTIO_MDEV_F_VERSION_1)
> =09vm_dev->version =3D 1;
> else
> =09//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 =3D ORED_LIST_OF_FEATURES;
> if (features & ~supported_features)
> =09//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.
>