linux-kernel - Re: [PATCH v5 5/5] Documentation: Add reconnect process for VDUSE

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Message-ID: <CACGkMEt3qpm=3Y=CDeA1Kyd7te-Wm-gk3j5iR96=_CaP0PrVtQ@mail.gmail.com>
Date: Tue, 16 Apr 2024 11:46:13 +0800
From: Jason Wang <jasowang@...hat.com>
To: Cindy Lu <lulu@...hat.com>
Cc: mst@...hat.com, linux-kernel@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v5 5/5] Documentation: Add reconnect process for VDUSE

On Fri, Apr 12, 2024 at 9:31 PM Cindy Lu <lulu@...hat.com> 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 <lulu@...hat.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..7faa83462e78 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 reconnection 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,

I guess you meant "exists"? If yes, it should be better to say "exits
unexpectedly"

> +   and we want to support its restart and connect to the kernel device again
> +
> +2. How can I support reconnection in a userspace application?

Better to say "How reconnection is supported"?

> +
> +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.

I think we need to mention that this should be part of the previous
"VDUSE devices are created as follows"?

> +
> +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.

Need to define "review" here and how to decide if it is not 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.

Not a native speaker, but it looks better with

"should be used by the userspace to store virtqueue specific information".

> 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.

I think we need some link/code examples to demonstrate what needs to be stored.

> When calling the ioctl VDUSE_VQ_GET_INFO to
> +    get vq information, it's necessary to check whether it's a reconnection.

Better with some examples of codes.

> 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

This seems to be unnecessary, for example there could be an unexpected exit.

Thanks

> --
> 2.43.0
>