Date:   Wed, 2 Dec 2020 17:21:54 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Parav Pandit <parav@...dia.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        "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

On Wed, Dec 2, 2020 at 12:53 PM Parav Pandit <parav@...dia.com> 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.

Yes, the backend need to be setup firstly, this is vendor device
specific, not vdpa specific.

> "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.
>

Yes, we can work like this. After we setup a backend through an
anonymous inode(fd) from /dev/vduse, we can get a unique handle. Then
use it to create a frontend which will connect to the specific
backend.

> 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?
>

How to create the vdpa block device? If we use the command "vdpa dev
add..", the command will hang there until a vduse process attaches to
the vdpa block device.

Thanks,
Yongji

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