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

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Message-ID: <20191119231023.GN4991@ziepe.ca>
Date:   Tue, 19 Nov 2019 19:10:23 -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 Tue, Nov 19, 2019 at 04:33:40PM -0500, Michael S. Tsirkin wrote:
> On Tue, Nov 19, 2019 at 03:15:47PM -0400, Jason Gunthorpe wrote:
> > On Tue, Nov 19, 2019 at 01:58:42PM -0500, Michael S. Tsirkin wrote:
> > > On Tue, Nov 19, 2019 at 12:46:32PM -0400, Jason Gunthorpe wrote:
> > > > As always, this is all very hard to tell without actually seeing real
> > > > accelerated drivers implement this. 
> > > > 
> > > > Your patch series might be a bit premature in this regard.
> > > 
> > > Actually drivers implementing this have been posted, haven't they?
> > > See e.g. https://lwn.net/Articles/804379/
> > 
> > Is that a real driver? It looks like another example quality
> > thing. 
> > 
> > For instance why do we need any of this if it has '#define
> > IFCVF_MDEV_LIMIT 1' ?
> > 
> > Surely for this HW just use vfio over the entire PCI function and be
> > done with it?
> 
> What this does is allow using it with unmodified virtio drivers
> within guests.  You won't get this with passthrough as it only
> implements parts of virtio in hardware.

I don't mean use vfio to perform passthrough, I mean to use vfio to
implement the software parts in userspace while vfio to talk to the
hardware.

  kernel -> vfio -> user space virtio driver -> qemu -> guest

Generally we don't want to see things in the kernel that can be done
in userspace, and to me, at least for this driver, this looks
completely solvable in userspace.

Jason