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

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Message-ID: <CACLfguVnQV1qaZeOXi5=T7KbGNEPibmUOXv_r8HZpcU0CMbgGQ@mail.gmail.com>
Date: Wed, 17 Apr 2024 16:46:44 +0800
From: Cindy Lu <lulu@...hat.com>
To: Jason Wang <jasowang@...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 Tue, Apr 16, 2024 at 11:46 AM Jason Wang <jasowang@...hat.com> wrote:
>
> 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
>
Thanks Jason, I will send a new version
Thanks
Cindy
> > --
> > 2.43.0
> >
>