Re: [PATCH] app/testpmd: add flag to enable RSS for single-queue

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Jul 22, 2025 at 07:08:15AM -0700, Stephen Hemminger wrote:
> On Tue, 15 Jul 2025 12:53:30 +0000
> Bruce Richardson <bruce.richardson@intel.com> wrote:
> 
> > For testing purposes it can be desirable to enable RSS even when only a
> > single queue per port is in use. Add an "enable-rss" flag to testpmd to
> > match the existing "disable-rss" flag.
> > 
> > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > ---
> 
> Not sure about this.
> It makes no sense to have RSS enabled with single queue.
> Maybe you want the hash but no steering?
> 

Yep, that is one scenario we have seen it being used in the past. Also,
it's a useful "harmless" offload that can be used to check that the offload
paths in the NIC are working correctly.

> But the DPDK RSS flag doesn't require driver to produce HASH.
> 
> I would even argue that ethdev layer should warn application
> trying to do single queue and RSS.

I definitely don't think it should warn about it, because there are
definitely usecases where you may want it enabled for single-queue case,
e.g. when using NIC that provides the hash. However, it should not be the
default, which is why this is a separate enabling flag.

Another reason it's good to have it, is to ensure that when testing single
queue vs multi-queue setups, we can test with the exact same configuration
between those two scenarios. Without the ability to enable RSS, then
multi-queue will always have a different NIC configuration to single queue.
In the case of some of our drivers, for example, the lack of any offloads
such as RSS causes a different faster RX path to be chosen in the
single-queue case, since no packet flags need to be handled.

/Bruce