[dpdk-dev] secondary processes and private data

[Stephen Hemminger]

On Wed, 26 Sep 2018 15:21:52 +0200
Thomas Monjalon <thomas at monjalon.net> wrote:

> Hi Alejandro,
> 
> 25/09/2018 16:10, Alejandro Lucero:
> > I've a problem when part of device private data needs to be private per
> > process.  
> 
> It appears we are facing the same issue to support multi-process in tap.
> 
> > Current multiprocess support shares device private data between primary and
> > secondaries but it is all dependent on a pointer initialized to the same
> > memory address by the multiprocess support code. If there is a per-process
> > data, If a secondary process changes it the primary gets affected, and the
> > same for additional secondaries which will affect not just the primary but
> > other previous secondaries.  
> 
> Yes, the field rte_eth_dev.data.dev_private is private to the device,
> but shared between processes.
> 
> > The solution is to add support for this inside struct rte_eth_dev,
> > something like
> > 
> > void *secondary_priv_data;
> > 
> > so it is up to the secondaries to use this field if necessary.  
> 
> I would say it is not only for secondary process.
> What about this name:
> 
> 	rte_eth_dev.process_private
> 
> > NFP PMD creates the required rte_eth_devs specifically, similar to what is
> > done inside rte_ethdev.c but adding initialization for an interface needed
> > when calling device ethdev_init function. There are other PMDs doing this
> > but none has this requirement for per-process private data.  
> 
> Actually tap has a per-process requirement for its file descriptors.
> 
> > Please, let me know what you think about this change to struct rte_ethdev
> > or if you have a better idea for solving this problem.  
> 
> I support the idea, but we need to agree on name bikeshedding :-)

Good idea, as long as it stays contained to DPDK. Don't want additional user API
pointers buried in internal structures (like ethdev).  If application needs device
private data it should manage its own state.