Subject: Re: [PATCH v3] Documentation: Add reconnect process for VDUSE

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 8 Apr 2024 08:50:43 -0400

On Mon, Apr 08, 2024 at 08:39:21PM +0800, Cindy Lu wrote:
> On Mon, Apr 8, 2024 at 3:40 PM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > On Thu, Apr 04, 2024 at 01:56:31PM +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..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,
> > > +   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.
> >
> > Confused. Where is that allocation, in code?
> >
> > Thanks!
> >
> this should allocated in function vduse_create_dev(),

I mean, it's not allocated there ATM right? This is just doc patch
to become part of a larger patchset?

> I will rewrite
> this part  to make it more clearer
> will send a new version soon
> Thanks
> cindy
> 
> > > +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
> >