Re: [RFC v3 08/11] vduse: Introduce VDUSE - vDPA Device in Userspace

[Randy Dunlap <rdunlap@infradead.org>]

Hi,

Documentation comments only:

On 1/18/21 9:07 PM, Xie Yongji wrote:
> 
> Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> ---
>  Documentation/driver-api/vduse.rst                 |   85 ++
> 
> diff --git a/Documentation/driver-api/vduse.rst b/Documentation/driver-api/vduse.rst
> new file mode 100644
> index 000000000000..9418a7f6646b
> --- /dev/null
> +++ b/Documentation/driver-api/vduse.rst
> @@ -0,0 +1,85 @@
> +==================================
> +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 VDUSE character device (/dev/vduse). Then a file descriptor pointing
> +to the new resources will be returned, which can be used to implement the
> +userspace vDPA device's control path and data path.
> +
> +To implement control path, the read/write operations to the file descriptor
> +will be used to receive/reply the control messages from/to VDUSE driver.
> +Those control messages are mostly based on the vdpa_config_ops which defines
> +a unified interface to control different types of vDPA device.
> +
> +The following types of messages are provided by the VDUSE framework now:
> +
> +- VDUSE_SET_VQ_ADDR: Set the addresses of the different aspects 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 (last_avail_idx) for virtqueue
> +
> +- VDUSE_GET_VQ_STATE: Get the state (last_avail_idx) 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
> +
> +Please see include/linux/vdpa.h for details.
> +
> +In the data path, vDPA device's iova regions will be mapped into userspace with
> +the help of VDUSE_IOTLB_GET_FD ioctl on the userspace vDPA device fd:
> +
> +- VDUSE_IOTLB_GET_FD: get the file descriptor to iova region. Userspace can
> +  access this iova region by passing the fd to mmap(2).
> +
> +Besides, the eventfd mechanism is used to trigger interrupt callbacks and
> +receive virtqueue kicks in userspace. The following ioctls on the userspace
> +vDPA device fd are provided to support that:
> +
> +- VDUSE_VQ_SETUP_KICKFD: set the kickfd for virtqueue, this eventfd is used
> +  by VDUSE driver to notify userspace to consume the vring.
> +
> +- VDUSE_VQ_SETUP_IRQFD: set the irqfd for virtqueue, this eventfd is used
> +  by userspace to notify VDUSE driver to trigger interrupt callbacks.
> +
> +MMU-based IOMMU Driver
> +----------------------
> +In virtio-vdpa case, VDUSE framework implements a MMU-based on-chip IOMMU

                                                   an MMU-based

> +driver to support mapping the kernel dma buffer into the userspace iova

                                        DMA

> +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.

       DMA

> +
> +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. During the mapping, unmapping, the driver will copy the data from
> +the original buffer to the bounce buffer and back, depending on the direction of
> +the transfer. And the bounce-buffer addresses will be mapped into the user address
> +space instead of the original one.


thanks.
-- 
~Randy