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From: Stephen Hemminger <stephen@networkplumber.org>
To: Thomas Monjalon <thomas@monjalon.net>
Cc: Shahaf Shuler <shahafs@mellanox.com>,
	"olivier.matz@6wind.com" <olivier.matz@6wind.com>,
	"dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] [PATCH] mbuf: extend pktmbuf pool private structure
Date: Tue, 19 Nov 2019 15:50:32 -0800
Message-ID: <20191119155032.78a0180c@hermes.lan> (raw)
In-Reply-To: <2817003.Afhorg6P4o@xps>

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?   
> > 
> > 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.

An example of how DPDK failed at this is the filter field in
rte_pdump. Since it is not checked for NULL, it can't safely
be used now (and still claim API/ABI compatiablity).

  reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-18 10:02 Shahaf Shuler
2019-11-18 16:12 ` Stephen Hemminger
2019-11-19  6:35   ` Shahaf Shuler
2019-11-19  9:32 ` Thomas Monjalon
2019-11-19 15:23   ` Shahaf Shuler
2019-11-19 16:25     ` Stephen Hemminger
2019-11-19 22:30       ` Thomas Monjalon
2019-11-19 23:50         ` Stephen Hemminger [this message]
2019-11-20  7:01           ` Shahaf Shuler
2019-11-20 15:56             ` Stephen Hemminger
2019-11-21 14:15               ` Olivier Matz
2019-11-21 12:28 ` [dpdk-dev] [PATCH v2] " Shahaf Shuler
2019-11-21 14:31   ` Olivier Matz
2019-11-24  5:52     ` Shahaf Shuler
2019-11-24  5:53   ` [dpdk-dev] [PATCH v3] " Shahaf Shuler
2019-11-24 17:50     ` Stephen Hemminger
2019-11-24 18:05       ` Thomas Monjalon
2019-11-24 18:10         ` Shahaf Shuler
2019-11-25  8:12     ` Olivier Matz
2019-11-25 10:21     ` [dpdk-dev] [PATCH v4] " Shahaf Shuler
2019-11-25 10:27       ` Olivier Matz
2019-11-25 21:42         ` Thomas Monjalon

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