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From: Cindy Lu <lulu@redhat.com>
Date: Thu, 4 Apr 2024 13:35:02 +0800
Message-ID: <CACLfguW5GxYLtgL-8itH6b9re25bGsorKBWwDW9z5kBucSPLVg@mail.gmail.com>
Subject: Re: [PATCH v2] Documentation: Add reconnect process for VDUSE
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: jasowang@redhat.com, virtualization@lists.linux-foundation.org, 
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org, netdev@vger.kernel.org
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On Fri, Mar 29, 2024 at 5:52=E2=80=AFPM Michael S. Tsirkin <mst@redhat.com>=
 wrote:
>
> 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 <lulu@redhat.com>
> > ---
> >  Documentation/userspace-api/vduse.rst | 41 +++++++++++++++++++++++++++
> >  1 file changed, 41 insertions(+)
> >
> > diff --git a/Documentation/userspace-api/vduse.rst b/Documentation/user=
space-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
>
Really sorry for this, I will send a new version

Thanks
Cindy
> > +------------------------------------
> > +1. What is reconnection?
> > +
> > +   When the userspace application loads, it should establish a connect=
ion
> > +   to the vduse kernel device. Sometimes,the userspace application exi=
sts,
> > +   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 veri=
fy the
> > +    existence of the device "/dev/vduse/vduse_name".
> > +    If it doesn't exist, it means this is the first-time for connectio=
n. 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 fo=
r
> > +    the reconnect information. The total memory size is PAGE_SIZE*vq_m=
umber.
> > +
> > +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_FE=
ATURES...)
> > +    to get the information from kernel.
> > +    Please review the information and confirm if it is suitable to rec=
onnect.
> > +
> > +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 runnin=
g. 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 relevan=
t 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 reconne=
ction. If it is
> > +    a reconnection, the vq-related information must be get from the ma=
pped 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?
>
We used to use tmpfs to save this reconnect information,
but this will make the API not self contained, so we changed to using
the kernel memory
Thanks
cindy


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