Subject: Re: [PATCH v2] Documentation: Add reconnect process for VDUSE

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 29 Mar 2024 05:51:08 -0400

On Fri, Mar 29, 2024 at 05:38:25PM +0800, Cindy Lu wrote:
> 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 <[email-masked]>
> ---
>  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

typo

> +------------------------------------
> +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.
> +

I don't get it. So this is just a way for the application to allocate
memory? Why do we need this new way to do it?
Why not just mmap a file anywhere at all?

> +2.6 When the Userspace application exits, it is necessary to unmap all the
> +    pages for reconnection
> -- 
> 2.43.0