RE: [PATCH v2] net/af_xdp: re-enable secondary process support

["Loftus, Ciara" <ciara.loftus@intel.com>]

> >>
> >> On 2/4/2022 12:54 PM, Ciara Loftus wrote:
> >>> Secondary process support had been disabled for the AF_XDP PMD
> >>> because there was no logic in place to share the AF_XDP socket
> >>> file descriptors between the processes. This commit introduces
> >>> this logic using the IPC APIs.
> >>>
> >>> Since AF_XDP rings are single-producer single-consumer, rx/tx
> >>> in the secondary process is disabled. However other operations
> >>> including retrieval of stats are permitted.
> >>>
> >>> Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
> >>>
> >>> ---
> >>> v1 -> v2:
> >>> * Rebase to next-net
> >>>
> >>> RFC -> v1:
> >>> * Added newline to af_xdp.rst
> >>> * Fixed spelling errors
> >>> * Fixed potential NULL dereference in init_internals
> >>> * Fixed potential free of address-of expression in
> afxdp_mp_request_fds
> >>> ---
> >>>    doc/guides/nics/af_xdp.rst             |   9 ++
> >>>    doc/guides/nics/features/af_xdp.ini    |   1 +
> >>>    doc/guides/rel_notes/release_22_03.rst |   1 +
> >>>    drivers/net/af_xdp/rte_eth_af_xdp.c    | 210
> >> +++++++++++++++++++++++--
> >>>    4 files changed, 207 insertions(+), 14 deletions(-)
> >>>
> >>> diff --git a/doc/guides/nics/af_xdp.rst b/doc/guides/nics/af_xdp.rst
> >>> index db02ea1984..eb4eab28a8 100644
> >>> --- a/doc/guides/nics/af_xdp.rst
> >>> +++ b/doc/guides/nics/af_xdp.rst
> >>> @@ -141,4 +141,13 @@ Limitations
> >>>      NAPI context from a watchdog timer instead of from softirqs. More
> >> information
> >>>      on this feature can be found at [1].
> >>>
> >>> +- **Secondary Processes**
> >>> +
> >>> +  Rx and Tx are not supported for secondary processes due to the
> single-
> >> producer
> >>> +  single-consumer nature of the AF_XDP rings. However other
> operations
> >> including
> >>> +  statistics retrieval are permitted.
> >>
> >> Hi Ciara,
> >>
> >> Isn't this limitation same for all PMDs, like not both primary & secondary
> can
> >> Rx/Tx
> >> from same queue at the same time.
> >> But primary can initiallize the PMD and secondary can do the datapath,
> >> or isn't af_xdp supports multiple queue, if so some queues can be used
> by
> >> primary and some by secondary for datapath.
> >>
> >> Is there anyhing special for af_xdp that prevents it?
> >
> > Hi Ferruh,
> >
> > Thanks for the review.
> > Each queue of the PMD corresponds to a new AF_XDP socket.
> > Each socket has an RX and TX ring that is mmapped from the kernel to
> userspace and this mapping is only valid for the primary process.
> > I did not figure out a way to share that mapping with the secondary process
> successfully. Can you think of anything that might work?
> >
> 
> Does the application knows the buffer address for the Rx/Tx, or is
> abstracted to the 'fd'?

The application knows the buffer address of the Rx/Tx rings.
We pass a pointer to these rings to the libbpf xsk_socket__create API, which sets up the mappings:
http://code.dpdk.org/dpdk/v21.11/source/drivers/net/af_xdp/rte_eth_af_xdp.c#L1291
Then later on in the datapath we operate directly on those rings:
http://code.dpdk.org/dpdk/v21.11/source/drivers/net/af_xdp/rte_eth_af_xdp.c#L268
The fd is used in the datapath, but just for the syscalls (recvfrom/poll/send).

> If only 'fd' is used, this patch already converts 'fd' between
> processes.
> cc'ed Anatoly, but what I understand is after MP fd conversion:
> Primary process: FD=x
> Secondary process: FD=y
> And both x & y points to exact same socket in the kernel side.
> 
> At least this is how it works for the 'tap' interface, and that is
> why 'fs' are in the process_private area and converted between primary
> and secondary, I thought it will be same for the xdp socket.
> 
> Did you test the secondary Rx/Tx in the secondary after this patch?