Re: [dpdk-dev] [PATCH] mbuf: extend pktmbuf pool private structure

[Olivier Matz <olivier.matz@6wind.com>]

Hi,

On Wed, Nov 20, 2019 at 07:56:14AM -0800, Stephen Hemminger wrote:
> On Wed, 20 Nov 2019 07:01:26 +0000
> Shahaf Shuler <shahafs@mellanox.com> wrote:
> 
> > Wednesday, November 20, 2019 1:51 AM, Stephen Hemminger:
> > > Subject: Re: [dpdk-dev] [PATCH] mbuf: extend pktmbuf pool private
> > > structure
> > > 
> > > On Tue, 19 Nov 2019 23:30:15 +0100
> > > Thomas Monjalon <thomas@monjalon.net> wrote:
> > >   
> > > > 19/11/2019 17:25, Stephen Hemminger:  
> > > > > On Tue, 19 Nov 2019 15:23:50 +0000
> > > > > Shahaf Shuler <shahafs@mellanox.com> wrote:
> > > > >  
> > > > > > Tuesday, November 19, 2019 11:33 AM, Thomas Monjalon:  
> > > > > > > Subject: Re: [PATCH] mbuf: extend pktmbuf pool private structure
> > > > > > >
> > > > > > > 18/11/2019 11:02, Shahaf Shuler:  
> > > > > > > >  struct rte_pktmbuf_pool_private {
> > > > > > > >  	uint16_t mbuf_data_room_size; /**< Size of data space in  
> > > each  
> > > > > > > mbuf. */  
> > > > > > > >  	uint16_t mbuf_priv_size;      /**< Size of private area in each  
> > > mbuf.  
> > > > > > > */  
> > > > > > > > +	uint32_t reserved; /**< reserved for future use. */  
> > > > > > >
> > > > > > > Maybe simpler to give the future name "flags" and keep the  
> > > comment  
> > > > > > > "reserved for future use".  
> > > > > >
> > > > > > I'm am OK w/ changing to flags.
> > > > > > If Olivier accepts maybe you can change while applying?  

OK for flags.

> > > > >
> > > > > After the Linux openat experience if you want to add flags.
> > > > > Then all usage of API needs to validate that flags is 0.  
> > > >
> > > > Sorry Stephen, I don't understand what you mean.
> > > > Please could you explain?
> > > >
> > > >  
> > > 
> > > Any time a new field is added that maybe used later you can not guarantee
> > > that existing code correctly initializes the value to zero. What happened with
> > > openat() was that there was a flag value that was originally unused, but since
> > > kernel did not enforce that it was zero; it could not later be used for
> > > extensions.
> > > 
> > > You need to make sure that all reserved fields are initialized.
> > > That means when a private pool is created it is zeroed. And if a flag is new
> > > argument to an API, check for zero at create time.  

+1, this is a good point

> > 
> > I guess we can hard code the value for 0 on the rte_pktmbuf_pool_create function and have some assert on the rte_pktmbuf_pool_init callback (we cannot fail as this function returns void).
> > Any other places you find problematic? 
> 
> No. that should be good. 

Adding an assertion in rte_pktmbuf_pool_init() to check that flag is 0
is a good idea.

In addition, we must ensure that mbp_priv->flags is set to 0 by calling
memset(&mbp_priv, 0, sizeof(mbp_priv)) at several places:

- in rte_pktmbuf_pool_init() when user_mbp_priv == NULL
- in rte_pktmbuf_pool_create_by_ops()
- in examples/ntb/ntb_fwd.c:ntb_mbuf_pool_create()

I think an entry in the release note could be added too.

Olivier