RE: vDPA bus driver selection

Hi Stefano,

From: Stefano Garzarella <sgarzare@redhat.com>
Sent: Wednesday, October 27, 2021 9:17 PM
To: Parav Pandit <redacted>
Cc: Jason Wang <redacted>; Michael Tsirkin <mst@redhat.com>;
Linux Virtualization [off-list ref]; Eli Cohen
[off-list ref]
Subject: Re: vDPA bus driver selection

Hi Parav,

On Wed, Oct 27, 2021 at 03:21:15PM +0000, Parav Pandit wrote:

quoted

Hi Stefano,

quoted

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. A use case could be for
example a single host handling VMs and bare-metal containers, so we
would have both virtio-vdpa and vhost-vdpa loaded and we want to
attach some devices to VMs through vhost-vdpa and others to containers

through virtio-vdpa.

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What do you think?

One option is, user keeps the drivers_autoprobe disabled for the vdpa
bus using,

$ vdpa/vdpa dev add mgmtdev vdpasim_net name vdpa0 mac
00:11:22:33:44:55 $ echo 0 > /sys/bus/vdpa/drivers_autoprobe

And after vdpa device creation, it manually binds to the desired driver
such as,

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

Cool, I didn't know that. This is very useful, but do you think it might be better
to integrate it with the netlink API and specify at creation which bus driver to
use?

I think it is useful; for vduse case we need the ability to say "none" as well and when nothing specified it should be default driver.

More than netlink, I think we need the ability in the core kernel to make this choice.
I haven't explored what is already available to make that happen.
If/once its available, I am sure it has more users than just vdpa.

quoted

In an case of VDUSE, it makes more sense to bind to the one of the
above driver after user space has connected the use space backend to
the kernel device.

Yep, make sense.

Thanks,
Stefano

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