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

["Xia, Chenbo" <chenbo.xia@intel.com>]

Hi Thomas,

> -----Original Message-----
> From: Thomas Monjalon <thomas@monjalon.net>
> Sent: Friday, June 11, 2021 3:16 PM
> To: Xia, Chenbo <chenbo.xia@intel.com>
> Cc: dev@dpdk.org; Liang, Cunming <cunming.liang@intel.com>; Wu, Jingjing
> <jingjing.wu@intel.com>; Burakov, Anatoly <anatoly.burakov@intel.com>; Yigit,
> Ferruh <ferruh.yigit@intel.com>; mdr@ashroe.eu; nhorman@tuxdriver.com;
> Richardson, Bruce <bruce.richardson@intel.com>; david.marchand@redhat.com;
> stephen@networkplumber.org; Ananyev, Konstantin <konstantin.ananyev@intel.com>
> Subject: Re: [dpdk-dev] [RFC v3 0/6] Add mdev (Mediated device) support in
> DPDK
> 
> 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.

> 
> > In this patchset, the PCI bus driver is extended to support scanning
> > and probing the mdev devices whose device-api is "vfio-pci".
> >
> >                      +---------+
> >                      | PCI bus |
> >                      +----+----+
> >                           |
> >          +--------+-------+-------+--------+
> >          |        |               |        |
> >   Physical PCI devices ...   Mediated PCI devices ...
> >
> > The first four patches in this patchset are mainly preparation of mdev
> > bus support. The left two patches are the key implementation of mdev bus.
> >
> > The implementation of mdev bus in DPDK has several options:
> >
> > 1: Embed mdev bus in current pci bus
> >
> >    This patchset takes this option for an example. Mdev has several
> >    device types: pci/platform/amba/ccw/ap. DPDK currently only cares
> >    pci devices in all mdev device types so we could embed the mdev bus
> >    into current pci bus. Then pci bus with mdev support will scan/plug/
> >    unplug/.. not only normal pci devices but also mediated pci devices.
> 
> I think it is a different bus.
> It would be cleaner to not touch the PCI bus.
> Having a separate bus will allow an easy way to identify a device
> with the new generic devargs syntax, example:
> 	bus=mdev,uuid=XXX
> or more complex:
> 	bus=mdev,uuid=XXX/class=crypto/driver=qat,foo=bar

OK. Agree on cleaner to not touch PCI bus. And there may also be a 'type=pci'
as mdev has several types in its definition (pci/ap/platform/ccw/...).

> 
> > 2: A new mdev bus that scans mediated pci devices and probes mdev driver to
> >    plug-in pci devices to pci bus
> >
> >    If we took this option, a new mdev bus will be implemented to scan
> >    mediated pci devices and a new mdev driver for pci devices will be
> >    implemented in pci bus to plug-in mediated pci devices to pci bus.
> >
> >    Our RFC v1 takes this option:
> >    http://patchwork.dpdk.org/project/dpdk/cover/20190403071844.21126-1-
> tiwei.bie@intel.com/
> >
> >    Note that: for either option 1 or 2, device drivers do not know the
> >    implementation difference but only use structs/functions exposed by
> >    pci bus. Mediated pci devices are different from normal pci devices
> >    on: 1. Mediated pci devices use UUID as address but normal ones use BDF.
> >    2. Mediated pci devices may have some capabilities that normal pci
> >    devices do not have. For example, mediated pci devices could have
> >    regions that have sparse mmap capability, which allows a region to have
> >    multiple mmap areas. Another example is mediated pci devices may have
> >    regions/part of regions not mmaped but need to access them. Above
> >    difference will change the current ABI (i.e., struct rte_pci_device).
> >    Please check 5th and 6th patch for details.
> >
> > 3. A brand new mdev bus that does everything
> >
> >    This option will implement a new and standalone mdev bus. This option
> >    does not need any changes in current pci bus but only needs some shared
> >    code (linux vfio part) in pci bus. Drivers of devices that support mdev
> >    will register itself as a mdev driver and do not rely on pci bus anymore.
> >    This option, IMHO, will make the code clean. The only potential problem
> >    may be code duplication, which could be solved by making code of linux
> >    vfio part of pci bus common and shared.
> 
> Yes I prefer this third option.
> We can find an elegant way of sharing some VFIO code between buses.

Yes, I have not thought about the details of the code sharing but will try to make
it elegant.

Thanks,
Chenbo

>