Date:   Wed, 2 Dec 2020 06:24:22 +0000
From:   Parav Pandit <parav@...dia.com>
To:     Jason Wang <jasowang@...hat.com>,
        Yongji Xie <xieyongji@...edance.com>
CC:     "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>, Eli Cohen <elic@...dia.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [External] Re: [PATCH 0/7] Introduce vdpa management tool



> From: Jason Wang <jasowang@...hat.com>
> Sent: Wednesday, December 2, 2020 11:21 AM
> 
> On 2020/12/2 下午12:53, Parav Pandit wrote:
> >
> >> From: Yongji Xie <xieyongji@...edance.com>
> >> Sent: Wednesday, December 2, 2020 9:00 AM
> >>
> >> On Tue, Dec 1, 2020 at 11:59 PM Parav Pandit <parav@...dia.com> wrote:
> >>>
> >>>
> >>>> From: Yongji Xie <xieyongji@...edance.com>
> >>>> Sent: Tuesday, December 1, 2020 7:49 PM
> >>>>
> >>>> On Tue, Dec 1, 2020 at 7:32 PM Parav Pandit <parav@...dia.com>
> wrote:
> >>>>>
> >>>>>
> >>>>>> From: Yongji Xie <xieyongji@...edance.com>
> >>>>>> Sent: Tuesday, December 1, 2020 3:26 PM
> >>>>>>
> >>>>>> On Tue, Dec 1, 2020 at 2:25 PM Jason Wang
> <jasowang@...hat.com>
> >>>> wrote:
> >>>>>>>
> >>>>>>> On 2020/11/30 下午3:07, Yongji Xie wrote:
> >>>>>>>>>> Thanks for adding me, Jason!
> >>>>>>>>>>
> >>>>>>>>>> Now I'm working on a v2 patchset for VDUSE (vDPA Device in
> >>>>>>>>>> Userspace) [1]. This tool is very useful for the vduse device.
> >>>>>>>>>> So I'm considering integrating this into my v2 patchset.
> >>>>>>>>>> But there is one problem：
> >>>>>>>>>>
> >>>>>>>>>> In this tool, vdpa device config action and enable action are
> >>>>>>>>>> combined into one netlink msg: VDPA_CMD_DEV_NEW. But in
> >>>> vduse
> >>>>>>>>>> case, it needs to be splitted because a chardev should be
> >>>>>>>>>> created and opened by a userspace process before we enable
> >>>>>>>>>> the vdpa device (call vdpa_register_device()).
> >>>>>>>>>>
> >>>>>>>>>> So I'd like to know whether it's possible (or have some
> >>>>>>>>>> plans) to add two new netlink msgs something like:
> >>>>>>>>>> VDPA_CMD_DEV_ENABLE
> >>>>>> and
> >>>>>>>>>> VDPA_CMD_DEV_DISABLE to make the config path more
> flexible.
> >>>>>>>>>>
> >>>>>>>>> Actually, we've discussed such intermediate step in some early
> >>>>>>>>> discussion. It looks to me VDUSE could be one of the users of
> >> this.
> >>>>>>>>> Or I wonder whether we can switch to use anonymous
> >>>>>>>>> inode(fd) for VDUSE then fetching it via an
> >>>>>>>>> VDUSE_GET_DEVICE_FD
> >> ioctl?
> >>>>>>>> Yes, we can. Actually the current implementation in VDUSE is
> >>>>>>>> like this.  But seems like this is still a intermediate step.
> >>>>>>>> The fd should be binded to a name or something else which need
> >>>>>>>> to be configured before.
> >>>>>>>
> >>>>>>> The name could be specified via the netlink. It looks to me the
> >>>>>>> real issue is that until the device is connected with a
> >>>>>>> userspace, it can't be used. So we also need to fail the
> >>>>>>> enabling if it doesn't
> >>>> opened.
> >>>>>> Yes, that's true. So you mean we can firstly try to fetch the fd
> >>>>>> binded to a name/vduse_id via an VDUSE_GET_DEVICE_FD, then
> use
> >>>>>> the name/vduse_id as a attribute to create vdpa device? It looks
> >>>>>> fine to
> >> me.
> >>>>> I probably do not well understand. I tried reading patch [1] and
> >>>>> few things
> >>>> do not look correct as below.
> >>>>> Creating the vdpa device on the bus device and destroying the
> >>>>> device from
> >>>> the workqueue seems unnecessary and racy.
> >>>>> It seems vduse driver needs
> >>>>> This is something should be done as part of the vdpa dev add
> >>>>> command,
> >>>> instead of connecting two sides separately and ensuring race free
> >>>> access to it.
> >>>>> So VDUSE_DEV_START and VDUSE_DEV_STOP should possibly be
> avoided.
> >>>>>
> >>>> Yes, we can avoid these two ioctls with the help of the management
> tool.
> >>>>
> >>>>> $ vdpa dev add parentdev vduse_mgmtdev type net name foo2
> >>>>>
> >>>>> When above command is executed it creates necessary vdpa device
> >>>>> foo2
> >>>> on the bus.
> >>>>> When user binds foo2 device with the vduse driver, in the probe(),
> >>>>> it
> >>>> creates respective char device to access it from user space.
> >>>>
> >>> I see. So vduse cannot work with any existing vdpa devices like ifc,
> >>> mlx5 or
> >> netdevsim.
> >>> It has its own implementation similar to fuse with its own backend of
> choice.
> >>> More below.
> >>>
> >>>> But vduse driver is not a vdpa bus driver. It works like vdpasim
> >>>> driver, but offloads the data plane and control plane to a user space
> process.
> >>> In that case to draw parallel lines,
> >>>
> >>> 1. netdevsim:
> >>> (a) create resources in kernel sw
> >>> (b) datapath simulates in kernel
> >>>
> >>> 2. ifc + mlx5 vdpa dev:
> >>> (a) creates resource in hw
> >>> (b) data path is in hw
> >>>
> >>> 3. vduse:
> >>> (a) creates resources in userspace sw
> >>> (b) data path is in user space.
> >>> hence creates data path resources for user space.
> >>> So char device is created, removed as result of vdpa device creation.
> >>>
> >>> For example,
> >>> $ vdpa dev add parentdev vduse_mgmtdev type net name foo2
> >>>
> >>> Above command will create char device for user space.
> >>>
> >>> Similar command for ifc/mlx5 would have created similar channel for
> >>> rest of
> >> the config commands in hw.
> >>> vduse channel = char device, eventfd etc.
> >>> ifc/mlx5 hw channel = bar, irq, command interface etc Netdev sim
> >>> channel = sw direct calls
> >>>
> >>> Does it make sense?
> >> In my understanding, to make vdpa work, we need a backend (datapath
> >> resources) and a frontend (a vdpa device attached to a vdpa bus). In
> >> the above example, it looks like we use the command "vdpa dev add ..."
> >>   to create a backend, so do we need another command to create a
> frontend?
> >>
> > For block device there is certainly some backend to process the IOs.
> > Sometimes backend to be setup first, before its front end is exposed.
> > "vdpa dev add" is the front end command who connects to the backend
> (implicitly) for network device.
> >
> > vhost->vdpa_block_device->backend_io_processor (usr,hw,kernel).
> >
> > And it needs a way to connect to backend when explicitly specified during
> creation time.
> > Something like,
> > $ vdpa dev add parentdev vdpa_vduse type block name foo3 handle
> <uuid>
> > In above example some vendor device specific unique handle is passed
> based on backend setup in hardware/user space.
> >
> > In below 3 examples, vdpa block simulator is connecting to backend block
> or file.
> >
> > $ vdpa dev add parentdev vdpa_blocksim type block name foo4 blockdev
> > /dev/zero
> >
> > $ vdpa dev add parentdev vdpa_blocksim type block name foo5 blockdev
> > /dev/sda2 size=100M offset=10M
> >
> > $ vdpa dev add parentdev vdpa_block filebackend_sim type block name
> > foo6 file /root/file_backend.txt
> >
> > Or may be backend connects to the created vdpa device is bound to the
> driver.
> > Can vduse attach to the created vdpa block device through the char device
> and establish the channel to receive IOs, and to setup the block config space?
> 
> 
> I think it can work.
> 
> Another thing I wonder it that, do we consider more than one VDUSE
> parentdev(or management dev)? This allows us to have separated devices
> implemented via different processes.
Multiple parentdev should be possible per one driver. for example mlx5_vdpa.ko will create multiple parent dev, one for each PCI VFs, SFs.
vdpa dev add can certainly use one parent/mgmt dev to create multiple vdpa devices.
Not sure why do we need to create multiple parent dev for that.
I guess there is just one parent/mgmt. dev for VDUSE. What will each mgmtdev do differently?
Demux of IOs, events will be per individual char dev level?

> 
> If yes, VDUSE ioctl needs to be extended to register/unregister parentdev.
> 
> Thanks
> 
> 
> >
> >> Thanks,
> >> Yongji

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