Issues around packet capture when secondary process is doing rx/tx

[Honnappa Nagarahalli]

> -----Original Message-----
> From: Stephen Hemminger <stephen at networkplumber.org>
> Sent: Sunday, January 7, 2024 7:59 PM
> To: dev at dpdk.org
> Cc: arshdeep.kaur at intel.com; Gowda, Sandesh <sandesh.gowda at intel.com>;
> Reshma Pattan <reshma.pattan at intel.com>
> Subject: Issues around packet capture when secondary process is doing rx/tx
> 
> I have been looking at a problem reported by Sandesh where packet capture
> does not work if rx/tx burst is done in secondary process.
> 
> The root cause is that existing rx/tx callback model just doesn't work unless the
> process doing the rx/tx burst calls is the same one that registered the callbacks.
This is not specific to packet capture. This is a generic problem and we should look to solve it generically.

> 
> An example sequence would be:
> 	1. dumpcap (or pdump) as secondary tells pdump in primary to register
> callback
> 	2. secondary process calls rx_burst.
> 	3. rx_burst sees the callback but it has pointer pdump_rx which is not
> necessarily
> 	   at same location in primary and secondary process.
> 	4. indirect function call in secondary to bad location likely causes crash.
> 
> Some possible workarounds.
> 	1. Keep callback list per-process: messy, but won't crash. Capture won't
> work
>            without other changes. In this primary would register callback, but
> secondaries
>            would not use them in rx/tx burst.
> 
> 	2. Replace use of rx/tx callback in pdump with change to rte_ethdev to
> have
>            a capture flag. (i.e. don't use indirection).  Likely ABI problems.
>            Basically, ignore the rx/tx callback mechanism. This is my preferred
> 	   solution.
> 
> 	3. Some fix up mechanism (in EAL mp support?) to have each process
> fixup
>            its callback mechanism.
Yes, would prefer this. Let the application call additional APIs to register the call backs in secondary process.

> 
> 	4. Do something in pdump_init to register the callback in same process
> context
> 	   (probably need callbacks to be per-process). Would mean callback is
> always
>            on independent of capture being enabled.
> 
>         5. Get rid of indirect function call pointer, and replace it by index into
>            a static table of callback functions. Every process would have same code
>            (in this case pdump_rx) but at different address.  Requires all callbacks
>            to be statically defined at build time.
> 
> The existing rx/tx callback is not safe id rx/tx burst is called from different
> process than where callback is registered.
>