Re: [PATCH V2 3/5] vDPA: introduce vDPA bus

On Fri, Feb 14, 2020 at 11:23:27AM +0800, Jason Wang wrote:

quoted

quoted

Though all vDPA devices have the same programming interface, but the
semantic is different. So it looks to me that use bus complies what
class.rst said:

"

Each device class defines a set of semantics and a programming interface
that devices of that class adhere to. Device drivers are the
implementation of that programming interface for a particular device on
a particular bus.

"

Here we are talking about the /dev/XX node that provides the
programming interface.

I'm confused here, are you suggesting to use class to create char device in
vhost-vdpa? That's fine but the comment should go for vhost-vdpa patch.

Certainly yes, something creating many char devs should have a
class. That makes the sysfs work as expected

I suppose this is vhost user? I admit I don't really see how this
vhost stuff works, all I see are global misc devices? Very unusual for
a new subsystem to be using global misc devices..

I would have expected that a single VDPA device comes out as a single
char dev linked to only that VDPA device.

quoted

All the vdpa devices have the same basic
chardev interface and discover any semantic variations 'in band'

That's not true, char interface is only used for vhost. Kernel virtio driver
does not need char dev but a device on the virtio bus.

Okay, this is fine, but why do you need two busses to accomplish this?

Shouldn't the 'struct virito_device' be the plug in point for HW
drivers I was talking about - and from there a vhost-user can connect
to the struct virtio_device to give it a char dev or a kernel driver
can connect to link it to another subsystem?

It is easy to see something is going wrong with this design because
the drivers/virtio/virtio_vdpa.c mainly contains a bunch of trampoline
functions reflecting identical calls from one ops struct to a
different ops struct. This suggests the 'vdpa' is some subclass of
'virtio' and it is possibly better to model it by extending 'struct
virito_device' to include the vdpa specific stuff.

Where does the vhost-user char dev get invovled in with the v2 series?
Is that included?

quoted

Every class of virtio traffic is going to need a special HW driver to
enable VDPA, that special driver can create the correct vhost side
class device.

Are you saying, e.g it's the charge of IFCVF driver to create vhost char dev
and other stuffs?

No.

Jason