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

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Date:   Fri, 14 Feb 2020 12:39:48 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jason Gunthorpe <jgg@...lanox.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     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 2020/2/14 上午12:13, Jason Gunthorpe wrote:
> On Thu, Feb 13, 2020 at 10:59:34AM -0500, Michael S. Tsirkin wrote:
>> On Thu, Feb 13, 2020 at 11:51:54AM -0400, Jason Gunthorpe wrote:
>>> The 'class' is supposed to provide all the library functions to remove
>>> this duplication. Instead of plugging the HW driver in via some bus
>>> scheme every subsystem has its own 'ops' that the HW driver provides
>>> to the subsystem's class via subsystem_register()
>> Hmm I'm not familiar with subsystem_register. A grep didn't find it
>> in the kernel either ...
> I mean it is the registration function provided by the subsystem that
> owns the class, for instance tpm_chip_register(),
> ib_register_device(), register_netdev(), rtc_register_device() etc
>
> So if you have some vhost (vhost net?) class then you'd have some
> vhost_vdpa_init/alloc(); vhost_vdpa_register(), sequence
> presumably. (vs trying to do it with a bus matcher)
>
> I recommend to look at rtc and tpm for fairly simple easy to follow
> patterns for creating a subsystem in the kernel. A subsystem owns a class,
> allows HW drivers to plug in to it, and provides a consistent user
> API via a cdev/sysfs/etc.
>
> The driver model class should revolve around the char dev and sysfs
> uABI - if you enumerate the devices on the class then they should all
> follow the char dev and sysfs interfaces contract of that class.
>
> Those examples show how to do all the refcounting semi-sanely,
> introduce sysfs, cdevs, etc.
>
> I thought the latest proposal was to use the existing vhost class and
> largely the existing vhost API, so it probably just needs to make sure
> the common class-wide stuff is split from the 'driver' stuff of the
> existing vhost to netdev.


Still, netdev is only one of the type we want to support. And we can not 
guarantee or forecast that vhost is the only API that is used.

Let's take virtio as an example, it is implemented through a bus which 
allows different subsystems on top. And it can provide a variety of 
different uAPIs. For best performance, VFIO could be used for userspace 
drivers, but it requires the bus has support from VFIO.

For vDPA devices, it's just the same logic. A bus allows different 
drivers and subsystems on top. One of the subsystem could be vhost that 
provides a unified API for userspace driver.

Thanks


>
> Jason
>