Re: vDPA bus driver selection

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Oct 29, 2021 at 10:31:22AM +0800, Jason Wang wrote:
>On Thu, Oct 28, 2021 at 5:47 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
>>
>> On Thu, Oct 28, 2021 at 10:24:47AM +0800, Jason Wang wrote:
>> >On Thu, Oct 28, 2021 at 4:16 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>> >>
>> >> On Wed, Oct 27, 2021 at 03:21:15PM +0000, Parav Pandit wrote:
>> >> > Hi Stefano,
>> >> >
>> >> > > From: Stefano Garzarella <sgarzare@redhat.com>
>> >> > > Sent: Wednesday, October 27, 2021 8:04 PM
>> >> > >
>> >> > > Hi folks,
>> >> > > I was trying to understand if we have a way to specify which vDPA bus driver
>> >> > > (e.g. vhost-vdpa, virtio-vdpa) a device should use.
>> >> > > IIUC we don't have it, and the first registered driver is used when a new device
>> >> > > is registered.
>> >> > >
>> >> > > I was thinking if it makes sense to extend the management API to specify which
>> >> > > bus driver to use for a device.
>> >
>> >Actually, we want to support this in the first version of vDPA bus.
>> >But for some reason it was dropped. The idea is to specify the device
>> >type 'virtio' or 'vhost'. But a concern is that, it may encourage
>> >vendor to implement e.g virtio specific device (without DMA
>> >isolation).
>>
>> Yep, I see the issue about device type, so I think make sense to require
>> the support of both, how it is now basically.
>>
>> So instead of defining the type of the device, we could provide the
>> possibility to choose which bus to connect it to,
>
>I think you meant the "bus driver" here?

Yep, sorry!

>
>> in this way we
>> continue to require that both are supported.
>>
>> As Michael suggested, instead of specify it at the creation time as 
>> was
>> in my original idea, we can provide an API to attach/detach a device 
>> to
>> a specific vDPA bus.
>
>Does such an API exist in driver core?

I need to check better, Parav showed something with sysfs to bind a 
device to a driver, so maybe yes.

I just tried the following and it worked:

$ vdpa dev add mgmtdev vdpasim_net name vdpa0

$ readlink -f /sys/bus/vdpa/devices/vdpa0/driver
/sys/bus/vdpa/drivers/vhost_vdpa

$ echo vdpa0 > /sys/bus/vdpa/devices/vdpa0/driver/unbind

$ echo vdpa0 > /sys/bus/vdpa/drivers/virtio_vdpa/bind

>
>>
>> Of course, providing a default behaviour like now, which connects to the
>> first registered.
>
>If we want to change this, we can introduce "driver_override".

Yep, I'll take a look.

Thanks,
Stefano

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