Re: [RFC v2 4/4] vduse: Add new ioctl VDUSE_GET_RECONNECT_INFO

[Maxime Coquelin]

On 9/25/23 04:57, Jason Wang wrote:
> On Thu, Sep 21, 2023 at 10:07 PM Cindy Lu <lulu@xxxxxxxxxx> wrote:
> > On Mon, Sep 18, 2023 at 4:49 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > > On Tue, Sep 12, 2023 at 11:01 AM Cindy Lu <lulu@xxxxxxxxxx> wrote:
> > > > In VDUSE_GET_RECONNECT_INFO, the Userspace App can get the map size
> > > > and The number of mapping memory pages from the kernel. The userspace
> > > > App can use this information to map the pages.
> > > >
> > > > Signed-off-by: Cindy Lu <lulu@xxxxxxxxxx>
> > > > ---
> > > >   drivers/vdpa/vdpa_user/vduse_dev.c | 15 +++++++++++++++
> > > >   include/uapi/linux/vduse.h         | 15 +++++++++++++++
> > > >   2 files changed, 30 insertions(+)
> > > >
> > > > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > > index 680b23dbdde2..c99f99892b5c 100644
> > > > --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> > > > +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > > @@ -1368,6 +1368,21 @@ static long vduse_dev_ioctl(struct file *file, unsigned int cmd,
> > > >                  ret = 0;
> > > >                  break;
> > > >          }
> > > > +       case VDUSE_GET_RECONNECT_INFO: {
> > > > +               struct vduse_reconnect_mmap_info info;
> > > > +
> > > > +               ret = -EFAULT;
> > > > +               if (copy_from_user(&info, argp, sizeof(info)))
> > > > +                       break;
> > > > +
> > > > +               info.size = PAGE_SIZE;
> > > > +               info.max_index = dev->vq_num + 1;
> > > > +
> > > > +               if (copy_to_user(argp, &info, sizeof(info)))
> > > > +                       break;
> > > > +               ret = 0;
> > > > +               break;
> > > > +       }
> > > >          default:
> > > >                  ret = -ENOIOCTLCMD;
> > > >                  break;
> > > > diff --git a/include/uapi/linux/vduse.h b/include/uapi/linux/vduse.h
> > > > index d585425803fd..ce55e34f63d7 100644
> > > > --- a/include/uapi/linux/vduse.h
> > > > +++ b/include/uapi/linux/vduse.h
> > > > @@ -356,4 +356,19 @@ struct vhost_reconnect_vring {
> > > >          _Bool avail_wrap_counter;
> > > >   };
> > > >
> > > > +/**
> > > > + * struct vduse_reconnect_mmap_info
> > > > + * @size: mapping memory size, always page_size here
> > > > + * @max_index: the number of pages allocated in kernel,just
> > > > + * use for check
> > > > + */
> > > > +
> > > > +struct vduse_reconnect_mmap_info {
> > > > +       __u32 size;
> > > > +       __u32 max_index;
> > > > +};
> > >
> > > One thing I didn't understand is that, aren't the things we used to
> > > store connection info belong to uAPI? If not, how can we make sure the
> > > connections work across different vendors/implementations. If yes,
> > > where?
> > >
> > > Thanks
> > The process for this reconnecttion  is
> > A.The first-time connection
> > 1> The userland app checks if the device exists
> > 2>  use the ioctl to create the vduse device
> > 3> Mapping the kernel page to userland and save the
> > App-version/features/other information to this page
> > 4>  if the Userland app needs to exit, then the Userland app will only
> > unmap the page and then exit
> >
> > B, the re-connection
> > 1> the userland app finds the device is existing
> > 2> Mapping the kernel page to userland
> > 3> check if the information in shared memory is satisfied to
> > reconnect,if ok then continue to reconnect
> > 4> continue working
> >
> >   For now these information are all from userland,So here the page will
> > be maintained by the userland App
> > in the previous code we only saved the api-version by uAPI .  if  we
> > need to support reconnection maybe we need to add 2 new uAPI for this,
> > one of the uAPI is to save the reconnect  information and another is
> > to get the information
> >
> > maybe something like
> >
> > struct vhost_reconnect_data {
> > uint32_t version;
> > uint64_t features;
> > uint8_t status;
> > struct virtio_net_config config;
> > uint32_t nr_vrings;
> > };
>
> Probably, then we can make sure the re-connection works across
> different vduse-daemon implementations.

+1, we need to have this defined in the uAPI to support interoperability
across different VDUSE userspace implementations.

> > #define VDUSE_GET_RECONNECT_INFO _IOR (VDUSE_BASE, 0x1c, struct
> > vhost_reconnect_data)
> >
> > #define VDUSE_SET_RECONNECT_INFO  _IOWR(VDUSE_BASE, 0x1d, struct
> > vhost_reconnect_data)
>
> Not sure I get this, but the idea is to map those pages to user space,
> any reason we need this uAPI?

It should not be necessary if the mmapped layout is properly defined.

Thanks,
Maxime

> Thanks
>
> > Thanks
> > Cindy
> >
> >
> >
> >
> > > > +
> > > > +#define VDUSE_GET_RECONNECT_INFO \
> > > > +       _IOWR(VDUSE_BASE, 0x1b, struct vduse_reconnect_mmap_info)
> > > > +
> > > >   #endif /* _UAPI_VDUSE_H_ */
> > > > --
> > > > 2.34.3