[PATCH V13 4/6] mdev: introduce mediated virtio bus

[Jason Gunthorpe]

On Wed, Nov 20, 2019 at 10:14:26AM +0800, Jason Wang wrote:

> > > I don't quite get the question here.
> > In the driver model the bus_type and foo_device are closely
> > linked.
> 
> I don't get the definition of "closely linked" here. Do you think the bus
> and device implement virtual bus series are closely linked? If yes, how did
> they achieve that?

I mean if you have a 'foo_device' then it should be on a 'foo_bus' and
not on some 'bar_bus', as that is how the driver core generally works.
 
> >   Creating 'mdev_device' instances and overriding the bus_type
> > is a very abusive thing to do.
> 
> Ok, mdev_device (without this series) had:
> 
> struct mdev_device {
>     struct device dev;
>     struct mdev_parent *parent;
>     guid_t uuid;
>     void *driver_data;
>     struct list_head next;
>     struct kobject *type_kobj;
>     struct device *iommu_device;
>     bool active;
> };
> 
> So it's nothing bus or VFIO specific. And what virtual bus had is:

What do mean? 'struct mdev_parent *parent' is the VFIO specific
stuff. I haven't figured out what the confusing mdev_parent is
supposed to be, or whhy the VFIO ops are linked to the parent or not
the device.. Honestly the whole mdev thing has a very strange take on
how to use the driver core.

> > Abusing it for other things is not appropriate. ie creating an
> > instance and not filling in most of the vfio focused ops is an abusive
> > thing to do.
> 
> Well, it's only half of the mdev_parent_ops in mdev_parent, various methods
> could be done do be more generic to avoid duplication of codes. No?

There are many ways to avoid duplicating code.

Taking something well defined, and bolting on something unrelated just
to share a bit of code is a very poor way to avoid code duplication.

> I'm sure you will need to handle other issues besides GUID which had been
> settled by mdev e.g IOMMU and types when starting to write a real hardware
> driver.

The iommu framework already handles that, the mdev stuff contributes
very little from what I can see.

> > Most likely, at least for virtio-net, everyone else will be able to
> > use devlink as well, making it much less clear if that GUID lifecycle
> > stuff is a good idea or not.
> 
> This assumption is wrong, we hard already had at least two concrete examples
> of vDPA device that doesn't use devlink:
> 
> - Intel IFC where virtio is done at VF level
> - Ali Cloud ECS instance where virtio is done at PF level

Again, you don't explain why they couldn't use devlink.

Or, why do we need GUID lifecycle stuff when these PCI devices can
only create a single virtio and can just go ahead and do that as soon
as they are probed.

The GUID stuff was invented for slicing, which you say is not
happening in these cases.

Jason