netdev - RE: [net-next v2 1/1] virtual-bus: Implementation of Virtual Bus

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Message-ID: <AM0PR05MB48663ADB0A470C78694F6B8DD14F0@AM0PR05MB4866.eurprd05.prod.outlook.com>
Date:   Wed, 20 Nov 2019 22:39:02 +0000
From:   Parav Pandit <parav@...lanox.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
CC:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        Dave Ertman <david.m.ertman@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "nhorman@...hat.com" <nhorman@...hat.com>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        Kiran Patil <kiran.patil@...el.com>,
        Tiwei Bie <tiwei.bie@...el.com>
Subject: RE: [net-next v2 1/1] virtual-bus: Implementation of Virtual Bus



> From: Alex Williamson <alex.williamson@...hat.com>
> Sent: Wednesday, November 20, 2019 4:08 PM
> 
> On Wed, 20 Nov 2019 14:11:08 -0400
> Jason Gunthorpe <jgg@...pe.ca> wrote:
> 
> > I feel like mdev is suffering from mission creep. I see people
> > proposing to use mdev for many wild things, the Mellanox SF stuff in
> > the other thread and this 'virtio subsystem' being the two that have
> > come up publicly this month.
> 
> Tell me about it... ;)
> 
Initial Mellanox sub function proposal was done using dedicated non-mdev subdev bus in [1] because mdev looked very vfio-ish.

Along the way mdev proposal was suggested at [2] by mdev maintainers to use.
The bus existed that detached two drivers (mdev and vfio_mdev), there was some motivation to attach other drivers.

After that we continued discussion and mdev extension using alias to have persistent naming in [3].

So far so good, but when we want to have actual use of mdev driver, it doesn't look right. :-)

> > Putting some boundaries on mdev usage would really help people know
> > when to use it. My top two from this discussion would be:
> >
> > - mdev devices should only bind to vfio. It is not a general kernel
> >   driver matcher mechanism. It is not 'virtual-bus'.
> 
So yes, we must define the scope of mdev and have right documentation to capture that.

If mdev is not supposed to be extended beyond vfio, why do you even need a bus? For iommu attachment?

[1] https://lkml.org/lkml/2019/3/1/19
[2] https://lkml.org/lkml/2019/3/7/696
[3] https://lkml.org/lkml/2019/8/26/854