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Date:   Fri, 14 Feb 2020 11:23:27 +0800
From:   Jason Wang <>
To:     Jason Gunthorpe <>
Subject: Re: [PATCH V2 3/5] vDPA: introduce vDPA bus

On 2020/2/13 下午11:05, Jason Gunthorpe wrote:
> On Thu, Feb 13, 2020 at 10:58:44PM +0800, Jason Wang wrote:
>> On 2020/2/13 下午9:41, 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.


> Jason

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