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

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 2/7/2022 7:49 AM, Loftus, Ciara wrote:
>>
>> 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'?
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?