Re: [dpdk-stable] [dpdk-dev] [PATCH] net/af_xdp: fix support of secondary process

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 20 Sep 2021 15:09:36 +0000
"Loftus, Ciara" <ciara.loftus@intel.com> wrote:

> > 
> > On Mon, 20 Sep 2021 13:23:57 +0000
> > "Loftus, Ciara" <ciara.loftus@intel.com> wrote:
> >   
> > > > -----Original Message-----
> > > > From: dev <dev-bounces@dpdk.org> On Behalf Of Stephen Hemminger
> > > > Sent: Friday 3 September 2021 17:15
> > > > To: dev@dpdk.org
> > > > Cc: Stephen Hemminger <stephen@networkplumber.org>;
> > > > stable@dpdk.org; xiaolong.ye@intel.com
> > > > Subject: [dpdk-dev] [PATCH] net/af_xdp: fix support of secondary  
> > process  
> > > >
> > > > Doing basic operations like info_get or get_stats was broken
> > > > in af_xdp PMD. The info_get would crash because dev->device
> > > > was NULL in secondary process. Fix this by doing same initialization
> > > > as af_packet and tap devices.
> > > >
> > > > The get_stats would crash because the XDP socket is not open in
> > > > primary process. As a workaround don't query kernel for dropped
> > > > packets when called from secondary process.
> > > >
> > > > Note: this does not address the other bug which is that transmitting
> > > > in secondary process is broken because the send() in tx_kick
> > > > will fail because XDP socket fd is not valid in secondary process.  
> > >
> > > Hi Stephen,
> > >
> > > Apologies for the delayed reply, I was on vacation.
> > >
> > > In the Bugzilla report you suggest we:
> > > "mark AF_XDP as broken in with primary/secondary
> > > and return an error in probe in secondary process".
> > > I agree with this suggestion. However with this patch we still permit  
> > secondary, and just make sure it doesn't crash for get_stats. Did you change
> > your mind?  
> > > Personally, I would prefer to have primary/secondary either working 100%  
> > or else not allowed at all by throwing an error during probe. What do you
> > think? Do you have a reason/use case to permit secondary processes despite
> > some features not being available eg. full stats, tx?  
> > >
> > > Thanks,
> > > Ciara  
> > 
> > There are two cases where secondary is useful even if send/receive can't
> > work from secondary process.
> > The pdump and proc-info applications can work with these patches.
> > 
> > I am using XDP over pdump as an easy way to get packets into the code for
> > testing.
> > 
> > The flag in the documentation doesn't have a "limited" version.
> > If you want, will send another patch to disable secondary support.  
> 
> Thanks for explaining. Since there are use cases for secondary, even if the functionality is limited, I don't think it should be disabled.
> Since we can't flag it as 'limited' in the feature matrix, could you please add a note about the send/receive limitation in the AF_XDP PMD documentation in a v2? There are already a number of limitations listed, which you can add to.
> 
> Thanks,
> Ciara
> 
> > 
> > Supporting secondary, means adding a mechanism to pass the socket
> > around.  

Looking at this in more detail, and my recommendation is:
For 21.11 release (and mark it as Fixes so it gets backported). Have the driver
return -ENOTSUP in secondary process.

For 22.03 add real secondary support using the rte_mp_msg to pass necessary
state to secondary process. Includes socket (and other memory regions?).

The pdump and proc-info cases are only useful for developer testing, and there are
other ways to do the same thing.