On Wed, Aug 05, 2020 at 10:18:41AM +0100, Kinsella, Ray wrote: > > > On 04/08/2020 18:18, Jerin Jacob wrote: > > On Tue, Aug 4, 2020 at 9:54 PM Bruce Richardson > > <bruce.richard...@intel.com> wrote: > >> > >> On Tue, Aug 04, 2020 at 09:33:14PM +0530, Jerin Jacob wrote: > >>> On Tue, Aug 4, 2020 at 7:55 PM Bruce Richardson > >>> <bruce.richard...@intel.com> wrote: > >>>> > >>>> On Tue, Aug 04, 2020 at 05:07:12PM +0530, Jerin Jacob wrote: > >>>>> On Tue, Aug 4, 2020 at 4:12 PM Bruce Richardson > >>>>> <bruce.richard...@intel.com> wrote: > >>>>>> > >>>>>> On Mon, Aug 03, 2020 at 12:59:03PM +0530, pbhagavat...@marvell.com > >>>>>> wrote: > >>>>>>> From: Pavan Nikhilesh <pbhagavat...@marvell.com> > >>>>>>> > >>>>>>> Add 64 byte padding at the end of event device public structure to > >>>>>>> allow > >>>>>>> future extensions. > >>>>>>> > >>>>>>> Signed-off-by: Pavan Nikhilesh <pbhagavat...@marvell.com> > >>>>>>> Acked-by: Jerin Jacob <jer...@marvell.com> > >>>>>>> --- > >>>>>>> v2 Changes: > >>>>>>> - Modify commit title. > >>>>>>> - Add patch reference to doc. > >>>>>>> > >>>>>>> doc/guides/rel_notes/deprecation.rst | 11 +++++++++++ > >>>>>>> 1 file changed, 11 insertions(+) > >>>>>>> > >>>>>>> diff --git a/doc/guides/rel_notes/deprecation.rst > >>>>>>> b/doc/guides/rel_notes/deprecation.rst > >>>>>>> index ea4cfa7a4..ec5db68e9 100644 > >>>>>>> --- a/doc/guides/rel_notes/deprecation.rst > >>>>>>> +++ b/doc/guides/rel_notes/deprecation.rst > >>>>>>> @@ -151,3 +151,14 @@ Deprecation Notices > >>>>>>> Python 2 support will be completely removed in 20.11. > >>>>>>> In 20.08, explicit deprecation warnings will be displayed when > >>>>>>> running > >>>>>>> scripts with Python 2. > >>>>>>> + > >>>>>>> +* eventdev: A 64 byte padding is added at the end of the following > >>>>>>> structures > >>>>>>> + in event device library to support future extensions: > >>>>>>> + ``rte_event_crypto_adapter_conf``, > >>>>>>> ``rte_event_eth_rx_adapter_conf``, > >>>>>>> + ``rte_event_eth_rx_adapter_queue_conf``, > >>>>>>> ``rte_event_eth_tx_adapter_conf``, > >>>>>>> + ``rte_event_timer_adapter_conf``, ``rte_event_timer_adapter_info``, > >>>>>>> + ``rte_event_dev_info``, ``rte_event_dev_config``, > >>>>>>> ``rte_event_queue_conf``, > >>>>>>> + ``rte_event_port_conf``, ``rte_event_timer_adapter``, > >>>>>>> + ``rte_event_timer_adapter_data``. > >>>>>>> + Reference: > >>>>>>> + > >>>>>>> http://patches.dpdk.org/project/dpdk/list/?series=10728&archive=both&state=* > >>>>>>> -- > >>>>>> > >>>>>> I don't like this idea of adding lots of padding to the ends of these > >>>>>> structures. For some structures, such as the public arrays for devices > >>>>>> it > >>>>>> may be necessary, but for all the conf structures passed as parameters > >>>>>> to > >>>>>> functions I think we can do better. Since these structures are passed > >>>>>> by > >>>>>> the user to various functions, function versioning can be used to > >>>>>> ensure > >>>>>> that the correct function in eventdev is always called. From there to > >>>>>> the > >>>>>> individual PMDs, we can implement ABI compatibility by either: > >>>>>> 1. including the length of the struct as a parameter to the driver. > >>>>>> (This is > >>>>>> a bit similar to my proposal for rawdev [1]) > >>>>>> 2. including the ABI version as a parameter to the driver. > >>>>> > >>>>> But, Will the above solution work if the application is dependent on > >>>>> struct size? > >>>>> i.e change of s1 size will change offset of s3 i.e > >>>>> app_sepecific_struct_s3. Right? > >>>>> i.e DPDK version should not change the offset of s3. Right? > >>>>> > >>>>> example, > >>>>> struct app_struct { > >>>>> struct dpdk_public_struct_s1 s1; > >>>>> struct dpdk_public_struct_s2 s2; > >>>>> struct app_sepecific_struct_s3 s3; > >>>>> } > >>>>> > >>>> Not sure what exactly you mean here. The actual offsets and sizes of the > >>>> structs will obviously change as you change the struct, but the end > >>>> compiled app has no idea of structs, all it knows of is offsets, which is > >>>> why you provide ABI compatible versions of the functions which use > >>>> "legacy" > >>>> copies of the structs to ensure correct offsets. It's pretty much > >>>> standard > >>>> practice for ABI versioning. > >>> > >>> Currently, We have only function versioning(not structure versioning). > >>> Are you suggesting having structure versioning? > >>> Will it complicate the code in terms of readability and supporting > >>> multiple > >>> structure versions aginst its support functions. > >>> > >> > >> We don't, and can't version structures, only functions are versioned. > >> Even if we do what you suggest and add a block of 64-bytes expansion room > >> at the end of the structures, how is the function you are calling expected > >> to know what the structure actually contains? For example, if you add a > >> field to the end, and reduce the padding by 8 bytes, your structure is > >> still the same size, and how does the called function know whether X or X+8 > >> bytes are valid in it. Basically, you still need to version all > >> functions using the structure, just as if you didn't bother extending the > >> struct. > > > > Yes. We need function versioning for sure if we change the behavior of > > the function. > > Is function version + reserved field enough to decode the correct value from > > the reserved filed from the structure. > > > > My concern is, Let say, we are making the change in structure a->b and > > function c->d > > assisted with it. > > > > In the reserved filed case: > > - struct a remains same(we will adding the fields in reserved filed) > > - the function will have c and d version and both using struct a > > > > In another scheme: > > - The application needs to change where versioned function(c or d) need to > > give associate structure manually. Right? If it is manually, it will > > be huge change > > in application. Right? > > > > How an application can express the correct structure mapping? > > Will it it be like > > rte_substrem_v21(struct rte_subsystem_conf_v21 *config)? > > vs > > rte_substrem_v21(struct rte_subsystem_conf *config)?> where > > rte_subsystem_conf has reserved filed. > > So the ABI policy approach for doing this is > > rte_substrem_v21(struct rte_subsystem_conf_v21 *config) > > instead of > > rte_substrem_v21(struct rte_subsystem_conf *config) (with extension padding). > > There are benefits and drawbacks with each approach, these include ... > > The padding approach assumes you are always happy to tack whatever field you > want > onto the end of the structure, when in many cases it's more natural home is > usually > in the middle or beginning. Then there is also dead/unused and uninitialized > memory, > to be conscious of. > > However what you say is completely correct, if I have a v21 version of the > function, > only it should be looking at the new field/additional bytes in the structure. > > The alternative is to version the structure along with the function. > And this is what is described in the ABI policy. > > The obvious drawback with this approach is that if you have a structure that > is used across > a large number of functions ... it can be a real headache as they all need to > be versioned. > > The benefit with this approach is that it is completely explicit, > which function and structure versions are associated. >
IMHO, the only time we should look to use padding is for structures like the ethdev structure that are internal-only but used across so many functions and inline functions that it simply becomes impossible to version them. For any structures that are simply passed as parameters, which I believe applies to all structs referred to by this deprecation notice, function versioning should be sufficient to deal with any issues. Regards, /Bruce
