Virtualization - Apr 2021 - [PATCH v6 10/10] Documentation: Add documentation for VDUSE

? 2021/3/31 ??4:05, Xie Yongji ??:
> 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 at bytedance.com>
> ---
>   Documentation/userspace-api/index.rst |   1 +
>   Documentation/userspace-api/vduse.rst | 212
++++++++++++++++++++++++++++++++++
>   2 files changed, 213 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..8c4e2b2df8bb
> --- /dev/null
> +++ b/Documentation/userspace-api/vduse.rst
> @@ -0,0 +1,212 @@
> +=================================> +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.
> +
> +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.request_id;
> +
> +		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 data 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 the first overlapped iova
region.
> +  Userspace can access this iova region by passing fd and corresponding
size, offset,
> +  perm to mmap(). For example:
> +
> +.. code-block:: c
> +
> +	static int perm_to_prot(uint8_t perm)
> +	{
> +		int prot = 0;
> +
> +		switch (perm) {
> +		case VDUSE_ACCESS_WO:
> +			prot |= PROT_WRITE;
> +			break;
> +		case VDUSE_ACCESS_RO:
> +			prot |= PROT_READ;
> +			break;
> +		case VDUSE_ACCESS_RW:
> +			prot |= PROT_READ | PROT_WRITE;
> +			break;
> +		}
> +
> +		return prot;
> +	}
> +
> +	static void *iova_to_va(int dev_fd, uint64_t iova, uint64_t *len)
> +	{
> +		int fd;
> +		void *addr;
> +		size_t size;
> +		struct vduse_iotlb_entry entry;
> +
> +		entry.start = iova;
> +		entry.last = iova + 1;
> +		fd = ioctl(dev_fd, VDUSE_IOTLB_GET_FD, &entry);
> +		if (fd < 0)
> +			return NULL;
> +
> +		size = entry.last - entry.start + 1;
> +		*len = entry.last - iova + 1;
> +		addr = mmap(0, size, perm_to_prot(entry.perm), MAP_SHARED,
> +			    fd, entry.offset);
> +		close(fd);
> +		if (addr == MAP_FAILED)
> +			return NULL;
> +
> +		/* do something to cache this iova region */
> +
> +		return addr + iova - entry.start;
> +	}
> +
> +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
> +
> +Register VDUSE device on vDPA bus
> +---------------------------------
> +In order to make the VDUSE device work, administrator needs to use the
management
> +API (netlink) to register it on vDPA bus. Some sample codes are show
below:
> +
> +.. code-block:: c
> +
> +	static int netlink_add_vduse(const char *name, int device_id)
> +	{
> +		struct nl_sock *nlsock;
> +		struct nl_msg *msg;
> +		int famid;
> +
> +		nlsock = nl_socket_alloc();
> +		if (!nlsock)
> +			return -ENOMEM;
> +
> +		if (genl_connect(nlsock))
> +			goto free_sock;
> +
> +		famid = genl_ctrl_resolve(nlsock, VDPA_GENL_NAME);
> +		if (famid < 0)
> +			goto close_sock;
> +
> +		msg = nlmsg_alloc();
> +		if (!msg)
> +			goto close_sock;
> +
> +		if (!genlmsg_put(msg, NL_AUTO_PORT, NL_AUTO_SEQ, famid, 0, 0,
> +		    VDPA_CMD_DEV_NEW, 0))
> +			goto nla_put_failure;
> +
> +		NLA_PUT_STRING(msg, VDPA_ATTR_DEV_NAME, name);
> +		NLA_PUT_STRING(msg, VDPA_ATTR_MGMTDEV_DEV_NAME, "vduse");
> +		NLA_PUT_U32(msg, VDPA_ATTR_DEV_ID, device_id);
> +
> +		if (nl_send_sync(nlsock, msg))
> +			goto close_sock;
> +
> +		nl_close(nlsock);
> +		nl_socket_free(nlsock);
> +
> +		return 0;
> +	nla_put_failure:
> +		nlmsg_free(msg);
> +	close_sock:
> +		nl_close(nlsock);
> +	free_sock:
> +		nl_socket_free(nlsock);
> +		return -1;
> +	}

Let's also explain this can be done via vdpa tool in iproute2 as well.

Otherwise

Acked-by: Jason Wang <jasowang at redhat.com>

> +
> +MMU-based IOMMU Driver
> +----------------------
> +VDUSE framework implements an MMU-based on-chip IOMMU driver to support
> +mapping the kernel DMA buffer into the userspace iova region dynamically.
> +This is mainly designed for virtio-vdpa case (kernel virtio drivers).
> +
> +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. 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.