[PATCH v2] Documentation: Add reconnect process for VDUSE

[Cindy Lu <lulu@redhat.com>]

Add a document explaining the reconnect process, including what the
Userspace App needs to do and how it works with the kernel.

Signed-off-by: Cindy Lu <lulu@redhat.com>
---
 Documentation/userspace-api/vduse.rst | 41 +++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/Documentation/userspace-api/vduse.rst b/Documentation/userspace-api/vduse.rst
index bdb880e01132..f903aed714d1 100644
--- a/Documentation/userspace-api/vduse.rst
+++ b/Documentation/userspace-api/vduse.rst
@@ -231,3 +231,44 @@ able to start the dataplane processing as follows:
    after the used ring is filled.
 
 For more details on the uAPI, please see include/uapi/linux/vduse.h.
+
+HOW VDUSE devices reconnectoin works
+------------------------------------
+1. What is reconnection?
+
+   When the userspace application loads, it should establish a connection
+   to the vduse kernel device. Sometimes,the userspace application exists,
+   and we want to support its restart and connect to the kernel device again
+
+2. How can I support reconnection in a userspace application?
+
+2.1 During initialization, the userspace application should first verify the
+    existence of the device "/dev/vduse/vduse_name".
+    If it doesn't exist, it means this is the first-time for connection. goto step 2.2
+    If it exists, it means this is a reconnection, and we should goto step 2.3
+
+2.2 Create a new VDUSE instance with ioctl(VDUSE_CREATE_DEV) on
+    /dev/vduse/control.
+    When ioctl(VDUSE_CREATE_DEV) is called, kernel allocates memory for
+    the reconnect information. The total memory size is PAGE_SIZE*vq_mumber.
+
+2.3 Check if the information is suitable for reconnect
+    If this is reconnection :
+    Before attempting to reconnect, The userspace application needs to use the
+    ioctl(VDUSE_DEV_GET_CONFIG, VDUSE_DEV_GET_STATUS, VDUSE_DEV_GET_FEATURES...)
+    to get the information from kernel.
+    Please review the information and confirm if it is suitable to reconnect.
+
+2.4 Userspace application needs to mmap the memory to userspace
+    The userspace application requires mapping one page for every vq. These pages
+    should be used to save vq-related information during system running. Additionally,
+    the application must define its own structure to store information for reconnection.
+
+2.5 Completed the initialization and running the application.
+    While the application is running, it is important to store relevant information
+    about reconnections in mapped pages. When calling the ioctl VDUSE_VQ_GET_INFO to
+    get vq information, it's necessary to check whether it's a reconnection. If it is
+    a reconnection, the vq-related information must be get from the mapped pages.
+
+2.6 When the Userspace application exits, it is necessary to unmap all the
+    pages for reconnection
-- 
2.43.0