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Date:   Mon, 21 Oct 2019 11:36:07 +0200
From:   Cornelia Huck <cohuck@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
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Subject: Re: [PATCH V4 5/6] virtio: introduce a mdev based transport
Message-ID: <20191021113607.16b26d9d.cohuck@redhat.com>
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        <20191018162007.31631039.cohuck@redhat.com>
        <2bb5645b-5c46-9cae-0571-65c302f51cf2@redhat.com>
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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
> > (...)
> > =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} =20
> > 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. =20
>=20
>=20
> 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 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

>=20
>=20
> >
> > 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? =20
>=20
>=20
> 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.

Ok, that makes sense.