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

[Stephen Hemminger]

On Mon, 8 Jan 2024 15:13:25 +0000
Konstantin Ananyev <konstantin.ananyev at huawei.com> wrote:

> > 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.
> > 
> > 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.  
> 
> As I remember, RX/TX callbacks were never intended to work over multiple processes.
> Right now RX/TX callbacks are private for the process, different process simply should not
> see/execute them.
> I.E. it callbacks list is part of 'struct rte_eth_dev' itself, not the rte_eth_dev.data that is shared
> between processes.
> It should be normal, wehn for the same port/queue you will end-up with different list of callbacks
> for different processes.  
> So, unless I am missing something, I don't see how we can end-up with 3) and 4) from above:
> From my understanding secondary process will never see/call primary's callbacks.
> 
> About pdump itself, it was a while when I looked at it last time, but as I remember to start it to work,
> server process has to call rte_pdump_init() which in terns register PDUMP_MP handler.
> I suppose for the secondary process to act as a 'pdump server' it needs to call rte_pdump_init() itself,
> though I am not sure such option is supported right now. 

Maybe the simplest would be just to make sure that rte_pdump_init() is called
in the process that does rx/tx burst. That might be made to work.
Still won't work for case where there are multiple secondary processes and some
the ethdev ports are used differently in each one, but would work better than now.