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

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Message-ID: <8b3e6a9c-8bfd-fb3c-12a8-2d6a3879f1ae@redhat.com>
Date:   Fri, 14 Feb 2020 11:23:27 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     mst@...hat.com, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
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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.

Thanks


>
> Jason
>