Re: [dpdk-dev] [PATCH] table: fix build error with gcc 8

["Dumitrescu, Cristian" <cristian.dumitrescu@intel.com>]

> -----Original Message-----
> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> Sent: Monday, April 9, 2018 4:10 PM
> To: Singh, Jasvinder <jasvinder.singh@intel.com>
> Cc: dev@dpdk.org; Dumitrescu, Cristian <cristian.dumitrescu@intel.com>
> Subject: Re: [dpdk-dev] [PATCH] table: fix build error with gcc 8
> 
> On Mon,  9 Apr 2018 13:49:48 +0100
> Jasvinder Singh <jasvinder.singh@intel.com> wrote:
> 
> > Fix build error with gcc 8.0 due to cast between function types.
> > Fixes: 5a80bf0ae613 ("table: add cuckoo hash")
> >
> > Signed-off-by: Jasvinder Singh <jasvinder.singh@intel.com>
> > ---
> >  lib/librte_table/rte_table_hash_cuckoo.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/lib/librte_table/rte_table_hash_cuckoo.c
> b/lib/librte_table/rte_table_hash_cuckoo.c
> > index dcb4fe9..f7eae27 100644
> > --- a/lib/librte_table/rte_table_hash_cuckoo.c
> > +++ b/lib/librte_table/rte_table_hash_cuckoo.c
> > @@ -103,11 +103,13 @@ rte_table_hash_cuckoo_create(void *params,
> >  		return NULL;
> >  	}
> >
> > +	void *hash_func = p->f_hash;
> > +
> >  	/* Create cuckoo hash table */
> >  	struct rte_hash_parameters hash_cuckoo_params = {
> >  		.entries = p->n_keys,
> >  		.key_len = p->key_size,
> > -		.hash_func = (rte_hash_function)(p->f_hash),
> > +		.hash_func = (rte_hash_function) hash_func,
> >  		.hash_func_init_val = p->seed,
> >  		.socket_id = socket_id,
> >  		.name = p->name
> 
> This is just tricking the compiler into not complaining.
> I would really rather see the two hash functions made the same.

(Adding Bruce as well to consolidate all conversations in a single thread.)

What we want to do here is be able to use the librte_hash under the same API as the several hash table flavors implemented in librte_table.

Both of these libraries allow configuring the hash function per each hash table instance. Problem is: hash function in librte_hash has only 3 parameters (no key mask), while hash function in librte_table has 4 parameters (includes key mask). The key mask helps a lot for practical protocol implementations by avoiding key copy & pre-process on lookup.

So then: how to plug in librte_hash under the same API as the suite of hash tables in librte_table? We don't want to re-implement cuckoo hash from librte_hash, we simply want to invoke it as a low-level primitive, similarly to how the LPM and ACL tables are plugged into librte_table.

Solution is: as an exception, pass a 3-parameter hash function to cuckoo hash flavor under the librte_table. Maybe this should be documented better. This currently triggers a build warning with gcc 8, which is easy to fix, hence this trivial patch.

Ideally, for every 3-parameter hash function, I would like to generate the corresponding 4-parameter hash function on-the-fly, but unfortunately this is not what C language can do.

Of course, IMO the best solution is to add key mask support to librte_hash.

Regards,
Cristian