Date:   Wed, 2 Dec 2020 21:18:16 +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 7:13 PM Parav Pandit <parav@...dia.com> wrote:
>
>
>
> > From: Yongji Xie <xieyongji@...edance.com>
> > Sent: Wednesday, December 2, 2020 2:52 PM
> >
> > 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.
>
> I do not fully understand the inode. But I assume this is some unique handle say uuid or something that both sides backend and vdpa device understand.
> It cannot be some kernel internal handle expose to user space.
>

Yes, the unique handle should be a user-defined stuff.

> >
> > > 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.
> I was suggesting that vdpa device is created, but it doesn’t have backend attached to it.
> It is attached to the backend when ioctl() side does enough setup. This state is handled internally the vduse driver.
>
> But the above method of preparing backend looks more sane.
>
> Regardless of which method is preferred, vduse driver must need a state to detach the vdpa bus device queues etc from the user space.
> This is needed because user space process can terminate anytime resulting in detaching dpa bus device in_use by the vhost side.

I think the vdpa device should only be detached by the command "vdpa
dev del...". The vduse driver can support reconnecting when user space
process is terminated.

Thanks,
Yongji

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