Re: Re: [RFC v4 09/11] Documentation: Add documentation for VDUSE

[Yongji Xie <xieyongji@bytedance.com>]

On Thu, Mar 4, 2021 at 2:40 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2021/2/23 7:50 下午, Xie Yongji wrote:
> > VDUSE (vDPA Device in Userspace) is a framework to support
> > implementing software-emulated vDPA devices in userspace. This
> > document is intended to clarify the VDUSE design and usage.
> >
> > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > ---
> >   Documentation/userspace-api/index.rst |   1 +
> >   Documentation/userspace-api/vduse.rst | 112 ++++++++++++++++++++++++++++++++++
> >   2 files changed, 113 insertions(+)
> >   create mode 100644 Documentation/userspace-api/vduse.rst
> >
> > diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
> > index acd2cc2a538d..f63119130898 100644
> > --- a/Documentation/userspace-api/index.rst
> > +++ b/Documentation/userspace-api/index.rst
> > @@ -24,6 +24,7 @@ place where this information is gathered.
> >      ioctl/index
> >      iommu
> >      media/index
> > +   vduse
> >
> >   .. only::  subproject and html
> >
> > diff --git a/Documentation/userspace-api/vduse.rst b/Documentation/userspace-api/vduse.rst
> > new file mode 100644
> > index 000000000000..2a20e686bb59
> > --- /dev/null
> > +++ b/Documentation/userspace-api/vduse.rst
> > @@ -0,0 +1,112 @@
> > +==================================
> > +VDUSE - "vDPA Device in Userspace"
> > +==================================
> > +
> > +vDPA (virtio data path acceleration) device is a device that uses a
> > +datapath which complies with the virtio specifications with vendor
> > +specific control path. vDPA devices can be both physically located on
> > +the hardware or emulated by software. VDUSE is a framework that makes it
> > +possible to implement software-emulated vDPA devices in userspace.
> > +
> > +How VDUSE works
> > +------------
> > +Each userspace vDPA device is created by the VDUSE_CREATE_DEV ioctl on
> > +the character device (/dev/vduse/control). Then a device file with the
> > +specified name (/dev/vduse/$NAME) will appear, which can be used to
> > +implement the userspace vDPA device's control path and data path.
>
>
> It's better to mention that in order to le thte device to be registered
> on the bus, admin need to use the management API(netlink) to create the
> vDPA device.
>
> Some codes to demnonstrate how to create the device will be better.
>

OK.

>
> > +
> > +To implement control path, a message-based communication protocol and some
> > +types of control messages are introduced in the VDUSE framework:
> > +
> > +- VDUSE_SET_VQ_ADDR: Set the vring address of virtqueue.
> > +
> > +- VDUSE_SET_VQ_NUM: Set the size of virtqueue
> > +
> > +- VDUSE_SET_VQ_READY: Set ready status of virtqueue
> > +
> > +- VDUSE_GET_VQ_READY: Get ready status of virtqueue
> > +
> > +- VDUSE_SET_VQ_STATE: Set the state for virtqueue
> > +
> > +- VDUSE_GET_VQ_STATE: Get the state for virtqueue
> > +
> > +- VDUSE_SET_FEATURES: Set virtio features supported by the driver
> > +
> > +- VDUSE_GET_FEATURES: Get virtio features supported by the device
> > +
> > +- VDUSE_SET_STATUS: Set the device status
> > +
> > +- VDUSE_GET_STATUS: Get the device status
> > +
> > +- VDUSE_SET_CONFIG: Write to device specific configuration space
> > +
> > +- VDUSE_GET_CONFIG: Read from device specific configuration space
> > +
> > +- VDUSE_UPDATE_IOTLB: Notify userspace to update the memory mapping in device IOTLB
> > +
> > +Those control messages are mostly based on the vdpa_config_ops in
> > +include/linux/vdpa.h which defines a unified interface to control
> > +different types of vdpa device. Userspace needs to read()/write()
> > +on the VDUSE device file to receive/reply those control messages
> > +from/to VDUSE kernel module as follows:
> > +
> > +.. code-block:: c
> > +
> > +     static int vduse_message_handler(int dev_fd)
> > +     {
> > +             int len;
> > +             struct vduse_dev_request req;
> > +             struct vduse_dev_response resp;
> > +
> > +             len = read(dev_fd, &req, sizeof(req));
> > +             if (len != sizeof(req))
> > +                     return -1;
> > +
> > +             resp.request_id = req.unique;
> > +
> > +             switch (req.type) {
> > +
> > +             /* handle different types of message */
> > +
> > +             }
> > +
> > +             len = write(dev_fd, &resp, sizeof(resp));
> > +             if (len != sizeof(resp))
> > +                     return -1;
> > +
> > +             return 0;
> > +     }
> > +
> > +In the deta path, vDPA device's iova regions will be mapped into userspace
> > +with the help of VDUSE_IOTLB_GET_FD ioctl on the VDUSE device file:
> > +
> > +- VDUSE_IOTLB_GET_FD: get the file descriptor to iova region. Userspace can
> > +  access this iova region by passing the fd to mmap().
>
>
> It would be better to have codes to explain how it is expected to work here.
>

OK.

>
> > +
> > +Besides, the following ioctls on the VDUSE device file are provided to support
> > +interrupt injection and setting up eventfd for virtqueue kicks:
> > +
> > +- VDUSE_VQ_SETUP_KICKFD: set the kickfd for virtqueue, this eventfd is used
> > +  by VDUSE kernel module to notify userspace to consume the vring.
> > +
> > +- VDUSE_INJECT_VQ_IRQ: inject an interrupt for specific virtqueue
> > +
> > +- VDUSE_INJECT_CONFIG_IRQ: inject a config interrupt
> > +
> > +MMU-based IOMMU Driver
> > +----------------------
> > +In virtio-vdpa case, VDUSE framework implements an MMU-based on-chip IOMMU
> > +driver to support mapping the kernel DMA buffer into the userspace iova
> > +region dynamically.
> > +
> > +The basic idea behind this driver is treating MMU (VA->PA) as IOMMU (IOVA->PA).
> > +The driver will set up MMU mapping instead of IOMMU mapping for the DMA transfer
> > +so that the userspace process is able to use its virtual address to access
> > +the DMA buffer in kernel.
> > +
> > +And to avoid security issue, a bounce-buffering mechanism is introduced to
> > +prevent userspace accessing the original buffer directly which may contain other
> > +kernel data.
>
>
> It's worth to mention this is designed for virtio-vdpa (kernel virtio
> drivers).
>

Will do it.

Thanks,
Yongji