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

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Date:   Thu, 13 Feb 2020 22:58:44 +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,
        tiwei.bie@...el.com, maxime.coquelin@...hat.com,
        cunming.liang@...el.com, zhihong.wang@...el.com,
        rob.miller@...adcom.com, xiao.w.wang@...el.com,
        haotian.wang@...ive.com, lingshan.zhu@...el.com,
        eperezma@...hat.com, lulu@...hat.com, parav@...lanox.com,
        kevin.tian@...el.com, stefanha@...hat.com, rdunlap@...radead.org,
        hch@...radead.org, aadam@...hat.com, jiri@...lanox.com,
        shahafs@...lanox.com, hanand@...inx.com, mhabets@...arflare.com
Subject: Re: [PATCH V2 3/5] vDPA: introduce vDPA bus


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.

"


>
> 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.


>
> 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.

Thanks

>
> I don't see a reason for VDPA to be creating busses..
>
> Jason
>