Re: [dpdk-dev] [RFC v3 0/6] Add mdev (Mediated device) support in DPDK

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jun 15, 2021 at 09:48:24AM +0200, Thomas Monjalon wrote:
> 15/06/2021 04:49, Xia, Chenbo:
> > From: Thomas Monjalon <thomas@monjalon.net>
> > > 01/06/2021 05:06, Chenbo Xia:
> > > > Hi everyone,
> > > >
> > > > This is a draft implementation of the mdev (Mediated device [1])
> > > > support in DPDK PCI bus driver. Mdev is a way to virtualize devices
> > > > in Linux kernel. Based on the device-api (mdev_type/device_api),
> > > > there could be different types of mdev devices (e.g. vfio-pci).
> > > 
> > > Please could you illustrate with an usage of mdev in DPDK?
> > > What does it enable which is not possible today?
> > 
> > The main purpose is for DPDK to drive mdev-based devices, which is not
> > possible today.
> > 
> > I'd take PCI devices for an example. Currently DPDK can only drive devices
> > of physical pci bus under /sys/bus/pci and kernel exposes the pci devices
> > to APP in that way.
> > 
> > But there are PCI devices using vfio-mdev as a software framework to expose
> > Mdev to APP under /sys/bus/mdev. Devices could choose this way of virtualizing
> > itself to let multiple APPs share one physical device. For example, Intel
> > Scalable IOV technology is known to use vfio-mdev as SW framework for Scalable
> > IOV enabled devices (and Intel net/crypto/raw devices support this tech). For
> > those mdev-based devices, DPDK needs support on the bus layer to scan/plug/probe/..
> > them, which is the main effort this patchset does. There are also other devices
> > using the vfio-mdev framework, AFAIK, Nvidia's GPU is the first one using mdev
> > and Intel's GPU virtualization also uses it.
> 
> Yes mdev was designed for virtualization I think.
> The use of mdev for Scalable IOV without virtualization
> may be seen as an abuse by Linux maintainers,
> as they currently seem to prefer the auxiliary bus (which is a real bus).
> 
> Mellanox got a push back when trying to use mdev for the same purpose
> (Scalable Function, also called Sub-Function) in the kernel.
> The Linux community decided to use the auxiliary bus.
> 
> Any other feedback on the choice mdev vs aux?
> Is there any kernel code supporting this mdev model for Intel devices?

IMHO until a kernel networking driver is accepted that uses mdev this
is all just dead code in dpdk and shouldn't be merged.

I think it is unlikely that future networking drivers will use mdev.

> > > > 2: A new mdev bus that scans mediated pci devices and probes mdev driver to
> > > >    plug-in pci devices to pci bus

And we are likely not doing 'mediated pci devices' at all..

Jason