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

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Message-ID: <20191120143054.GF22515@ziepe.ca>
Date:   Wed, 20 Nov 2019 10:30:54 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        Parav Pandit <parav@...lanox.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>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Bie, Tiwei" <tiwei.bie@...el.com>
Subject: Re: [net-next v2 1/1] virtual-bus: Implementation of Virtual Bus

On Wed, Nov 20, 2019 at 08:43:20AM -0500, Michael S. Tsirkin wrote:
> On Wed, Nov 20, 2019 at 09:03:19AM -0400, Jason Gunthorpe wrote:
> > On Wed, Nov 20, 2019 at 02:38:08AM -0500, Michael S. Tsirkin wrote:
> > > > > I don't think that extends as far as actively encouraging userspace
> > > > > drivers poking at hardware in a vendor specific way.  
> > > > 
> > > > Yes, it does, if you can implement your user space requirements using
> > > > vfio then why do you need a kernel driver?
> > > 
> > > People's requirements differ. You are happy with just pass through a VF
> > > you can already use it. Case closed. There are enough people who have
> > > a fixed userspace that people have built virtio accelerators,
> > > now there's value in supporting that, and a vendor specific
> > > userspace blob is not supporting that requirement.
> > 
> > I have no idea what you are trying to explain here. I'm not advocating
> > for vfio pass through.
> 
> You seem to come from an RDMA background, used to userspace linking to
> vendor libraries to do basic things like push bits out on the network,
> because users live on the performance edge and rebuild their
> userspace often anyway.
> 
> Lots of people are not like that, they would rather have the
> vendor-specific driver live in the kernel, with userspace being
> portable, thank you very much.

You are actually proposing a very RDMA like approach with a split
kernel/user driver design. Maybe the virtio user driver will turn out
to be 'portable'.

Based on the last 20 years of experience, the kernel component has
proven to be the larger burden and drag than the userspace part. I
think the high interest in DPDK, SPDK and others show this is a common
principle.

At the very least for new approaches like this it makes alot of sense
to have a user space driver until enough HW is available that a
proper, well thought out kernel side can be built.

For instance, this VFIO based approach might be very suitable to the
intel VF based ICF driver, but we don't yet have an example of non-VF
HW that might not be well suited to VFIO.

Jason