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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Dumitrescu, Cristian
> Sent: Monday, April 9, 2018 6:02 PM
> To: Van Haaren, Harry <harry.van.haaren@intel.com>; Stephen Hemminger <stephen@networkplumber.org>; Singh, Jasvinder
> <jasvinder.singh@intel.com>; Richardson, Bruce <bruce.richardson@intel.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] table: fix build error with gcc 8
> 
> 
> 
> > -----Original Message-----
> > From: Van Haaren, Harry
> > Sent: Monday, April 9, 2018 5:38 PM
> > To: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>; Stephen
> > Hemminger <stephen@networkplumber.org>; Singh, Jasvinder
> > <jasvinder.singh@intel.com>; Richardson, Bruce
> > <bruce.richardson@intel.com>
> > Cc: dev@dpdk.org
> > Subject: RE: [dpdk-dev] [PATCH] table: fix build error with gcc 8
> >
> > > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Dumitrescu,
> > Cristian
> > > Sent: Monday, April 9, 2018 4:59 PM
> > > To: Stephen Hemminger <stephen@networkplumber.org>; Singh,
> > Jasvinder
> > > <jasvinder.singh@intel.com>; Richardson, Bruce
> > <bruce.richardson@intel.com>
> > > Cc: dev@dpdk.org
> > > Subject: Re: [dpdk-dev] [PATCH] table: fix build error with gcc 8
> > >
> > >
> > >
> > > > -----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.
> >
> >
> > Looking at the previous discussion I see the following as a possible solution;
> >
> > Given the current code looks broken it should be fixed in this release.
> 
> The code is not broken. This is not a bug, it is a limitation for that particular table type. The only gap that I see is adding a Doxygen
> comment in the API header file.
> 
> User explicitly picks the hash table type it wants; when using this particular hash table type, that pointer needs to point to a 3-parameter
> function instead of 4. Given the limitation is clearly documented in Doxygen (current gap that we can quickly address), I don't see any
> problem.
> 
> If people think that this function conversion is not nice, it can be reworked in multiple ways at the expense of API (but not ABI) change:
> 1. Define the hash function field in the table parameter structure as opaque void * rather than 4-parameter version.
> 2. Create a separate parameter structure just for this hash table type.

Why just not define your f_hash member as a union:

struct rte_table_hash_params {
...
union {
    rte_table_hash_op_hash  f_hash_4params;
    rte_hash_function f_hash_3_params;
}; 

?

> 
> > Given the actual code fix is an API / ABI break (depending on solution) it
> > cannot be merged official in this release.
> > We have a NEXT_ABI macro - it allows us to break API/ABI conditionally at
> > compile time.
> 
> This is not new code introduced in this release cycle, this is just fixing the compiler warning, I fail to see how your ABI breakage mention is
> applicable.
> 
> Maybe we should talk more specifics over the code, where exactly in the code would you like to place your NEXT_ABI macro?
> 
> >
> > With the above 3 points, I think the best solution is to correctly fix the
> > problem that GCC 8 is identifying, and putting that new API inside the NEXT_
> > macros.
> >
> > In this case, we can preserve backwards (buggy) behavior if required, and
> > provide correct (but API/ABI breaking) code as well. This is a tough decision -
> > particularly for distros - what do they package?
> >
> > Given the current code, I don't see a better solution - but I hope I'm wrong :)