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

[Jason Wang]

On 2020/2/13 äå11:05, Jason Gunthorpe wrote:
> On Thu, Feb 13, 2020 at 10:58:44PM +0800, Jason Wang wrote:
> > On 2020/2/13 äå9:41, Jason Gunthorpe wrote:
> > > On Thu, Feb 13, 2020 at 11:34:10AM +0800, Jason Wang wrote:
> > >
> > > > >     You have dev, type or
> > > > > class to choose from. Type is rarely used and doesn't seem to be used
> > > > > by vdpa, so class seems the right choice
> > > > >
> > > > > Jason
> > > > Yes, but my understanding is class and bus are mutually exclusive. So we
> > > > can't add a class to a device which is already attached on a bus.
> > > While I suppose there are variations, typically 'class' devices are
> > > user facing things and 'bus' devices are internal facing (ie like a
> > > PCI device)
> >
> > 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.


> 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.


> > > So why is this using a bus? VDPA is a user facing object, so the
> > > driver should create a class vhost_vdpa device directly, and that
> > > driver should live in the drivers/vhost/ directory.
> >   
> > This is because we want vDPA to be generic for being used by different
> > drivers which is not limited to vhost-vdpa. E.g in this series, it allows
> > vDPA to be used by kernel virtio drivers. And in the future, we will
> > probably introduce more drivers in the future.
> I don't see how that connects with using a bus.


This is demonstrated in the virito-vdpa driver. So if you want to use 
kernel virito driver for vDPA device, a bus is most straight forward.


> 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?


> > > For the PCI VF case this driver would bind to a PCI device like
> > > everything else
> > >
> > > For our future SF/ADI cases the driver would bind to some
> > > SF/ADI/whatever device on a bus.
> > All these driver will still be bound to their own bus (PCI or other). And
> > what the driver needs is to present a vDPA device to virtual vDPA bus on
> > top.
> Again, I can't see any reason to inject a 'vdpa virtual bus' on
> top. That seems like mis-using the driver core.


I don't think so. Vhost is not the only programming interface for vDPA. 
We don't want a device that can only work for userspace drivers and only 
have a single set of userspace APIs.

Thanks


> Jason