Re: [PATCH V3 4/7] mdev: introduce device specific ops

Alex Williamson
2019-10-15T17:26:46Z

On Tue, 15 Oct 2019 20:17:01 +0800
Jason Wang <jasowang@redhat.com> wrote:

> On 2019/10/15 下午6:41, Cornelia Huck wrote:
> > On Fri, 11 Oct 2019 16:15:54 +0800
> > Jason Wang <jasowang@redhat.com> wrote:
> >
> >> Currently, except for the create and remove, the rest of
> >> mdev_parent_ops is designed for vfio-mdev driver only and may not help
> >> for kernel mdev driver. With the help of class id, this patch
> >> introduces device specific callbacks inside mdev_device
> >> structure. This allows different set of callback to be used by
> >> vfio-mdev and virtio-mdev.
> >>
> >> Signed-off-by: Jason Wang <jasowang@redhat.com>
> >> ---
> >> .../driver-api/vfio-mediated-device.rst | 22 +++++---
> >> MAINTAINERS | 1 +
> >> drivers/gpu/drm/i915/gvt/kvmgt.c | 18 ++++---
> >> drivers/s390/cio/vfio_ccw_ops.c | 18 ++++---
> >> drivers/s390/crypto/vfio_ap_ops.c | 14 +++--
> >> drivers/vfio/mdev/mdev_core.c | 9 +++-
> >> drivers/vfio/mdev/mdev_private.h | 1 +
> >> drivers/vfio/mdev/vfio_mdev.c | 37 ++++++-------
> >> include/linux/mdev.h | 42 +++------------
> >> include/linux/vfio_mdev.h | 52 +++++++++++++++++++
> >> samples/vfio-mdev/mbochs.c | 20 ++++---
> >> samples/vfio-mdev/mdpy.c | 21 +++++---
> >> samples/vfio-mdev/mtty.c | 18 ++++---
> >> 13 files changed, 177 insertions(+), 96 deletions(-)
> >> create mode 100644 include/linux/vfio_mdev.h
> >>
> >> diff --git a/Documentation/driver-api/vfio-mediated-device.rst b/Documentation/driver-api/vfio-mediated-device.rst
> >> index 2035e48da7b2..553574ebba73 100644
> >> --- a/Documentation/driver-api/vfio-mediated-device.rst
> >> +++ b/Documentation/driver-api/vfio-mediated-device.rst
> >> @@ -152,11 +152,20 @@ callbacks per mdev parent device, per mdev type, or any other categorization.
> >> Vendor drivers are expected to be fully asynchronous in this respect or
> >> provide their own internal resource protection.)
> >>
> >> -The callbacks in the mdev_parent_ops structure are as follows:
> >> +In order to support multiple types of device/driver, device needs to
> >> +provide both class_id and device_ops through:
> > "As multiple types of mediated devices may be supported, the device
> > needs to set up the class id and the device specific callbacks via:"
> >
> > ?
> >
> >>
> >> -* open: open callback of mediated device
> >> -* close: close callback of mediated device
> >> -* ioctl: ioctl callback of mediated device
> >> + void mdev_set_class(struct mdev_device *mdev, u16 id, const void *ops);
> >> +
> >> +The class_id is used to be paired with ids in id_table in mdev_driver
> >> +structure for probing the correct driver.
> > "The class id (specified in id) is used to match a device with an mdev
> > driver via its id table."
> >
> > ?
> >
> >> The device_ops is device
> >> +specific callbacks which can be get through mdev_get_dev_ops()
> >> +function by mdev bus driver.
> > "The device specific callbacks (specified in *ops) are obtainable via
> > mdev_get_dev_ops() (for use by the mdev bus driver.)"
> >
> > ?
> >
> >> For vfio-mdev device, its device specific
> >> +ops are as follows:
> > "A vfio-mdev device (class id MDEV_ID_VFIO) uses the following
> > device-specific ops:"
> >
> > ?
>
>
> All you propose is better than what I wrote, will change the docs.
>
>
> >
> >> +
> >> +* open: open callback of vfio mediated device
> >> +* close: close callback of vfio mediated device
> >> +* ioctl: ioctl callback of vfio mediated device
> >> * read : read emulation callback
> >> * write: write emulation callback
> >> * mmap: mmap emulation callback
> >> @@ -167,9 +176,10 @@ register itself with the mdev core driver::
> >> extern int mdev_register_device(struct device *dev,
> >> const struct mdev_parent_ops *ops);
> >>
> >> -It is also required to specify the class_id through::
> >> +It is also required to specify the class_id and device specific ops through::
> >>
> >> - extern int mdev_set_class(struct device *dev, u16 id);
> >> + extern int mdev_set_class(struct device *dev, u16 id,
> >> + const void *ops);
> > Apologies if that has already been discussed, but do we want a 1:1
> > relationship between id and ops, or can different devices with the same
> > id register different ops?
>
>
> I think we have a N:1 mapping between id and ops, e.g we want both
> virtio-mdev and vhost-mdev use a single set of device ops.

The contents of the ops structure is essentially defined by the id,
which is why I was leaning towards them being defined together. They
are effectively interlocked, the id defines which mdev "endpoint"
driver is loaded and that driver requires mdev_get_dev_ops() to return
the structure required by the driver. I wish there was a way we could
incorporate type checking here. We toyed with the idea of having the
class in the same structure as the ops, but I think this approach was
chosen for simplicity. We could still do something like:

int mdev_set_class_struct(struct device *dev, const struct mdev_class_struct *class);

struct mdev_class_struct {
u16 id;
union {
struct vfio_mdev_ops vfio_ops;
struct virtio_mdev_ops virtio_ops;
};
};

Maybe even:

struct vfio_mdev_ops *mdev_get_vfio_ops(struct mdev_device *mdev) {
BUG_ON(mdev->class.id != MDEV_ID_VFIO);
return &mdev->class.vfio_ops;
}

The match callback would of course just use the mdev->class.id value.
Functionally equivalent, but maybe better type characteristics. Thanks,

Alex