[dpdk-dev] [PATCH v1 3/9] mempool: remove callback to get capabilities

[Andrew Rybchenko]

On 03/15/2018 03:00 PM, Burakov, Anatoly wrote:
> On 15-Mar-18 11:49 AM, Andrew Rybchenko wrote:
>> On 03/15/2018 12:48 PM, Burakov, Anatoly wrote:
>>> On 14-Mar-18 5:24 PM, Andrew Rybchenko wrote:
>>>> On 03/14/2018 07:53 PM, Burakov, Anatoly wrote:
>>>>> On 14-Mar-18 4:12 PM, Andrew Rybchenko wrote:
>>>>>> On 03/14/2018 05:40 PM, Burakov, Anatoly wrote:
>>>>>>> On 10-Mar-18 3:39 PM, Andrew Rybchenko wrote:
>>>>>>>> The callback was introduced to let generic code to know octeontx
>>>>>>>> mempool driver requirements to use single physically contiguous
>>>>>>>> memory chunk to store all objects and align object address to
>>>>>>>> total object size. Now these requirements are met using a new
>>>>>>>> callbacks to calculate required memory chunk size and to populate
>>>>>>>> objects using provided memory chunk.
>>>>>>>>
>>>>>>>> These capability flags are not used anywhere else.
>>>>>>>>
>>>>>>>> Restricting capabilities to flags is not generic and likely to
>>>>>>>> be insufficient to describe mempool driver features. If required
>>>>>>>> in the future, API which returns structured information may be
>>>>>>>> added.
>>>>>>>>
>>>>>>>> Signed-off-by: Andrew Rybchenko <arybchenko at solarflare.com>
>>>>>>>> ---
>>>>>>>
>>>>>>> Just a general comment - it is not enough to describe minimum 
>>>>>>> memchunk requirements. With memory hotplug patchset that's 
>>>>>>> hopefully getting merged in 18.05, memzones will no longer be 
>>>>>>> guaranteed to be IOVA-contiguous. So, if a driver requires its 
>>>>>>> mempool to not only be populated from a single memzone, but a 
>>>>>>> single *physically contiguous* memzone, going by only callbacks 
>>>>>>> will not do, because whether or not something should be a single 
>>>>>>> memzone says nothing about whether this memzone has to also be 
>>>>>>> IOVA-contiguous.
>>>>>>>
>>>>>>> So i believe this needs to stay in one form or another.
>>>>>>>
>>>>>>> (also it would be nice to have a flag that a user could pass to 
>>>>>>> mempool_create that would force memzone reservation be 
>>>>>>> IOVA-contiguous, but that's a topic for another conversation. 
>>>>>>> prime user for this would be KNI.)
>>>>>>
>>>>>> I think that min_chunk_size should be treated as IOVA-contiguous.
>>>>>
>>>>> Why? It's perfectly reasonable to e.g. implement a software 
>>>>> mempool driver that would perform some optimizations due to all 
>>>>> objects being in the same VA-contiguous memzone, yet not be 
>>>>> dependent on underlying physical memory layout. These are two 
>>>>> separate concerns IMO.
>>>>
>>>> It looks like there is some misunderstanding here or I simply don't 
>>>> understand your point.
>>>> Above I mean that driver should be able to advertise its 
>>>> requirements on IOVA-contiguous regions.
>>>> If driver do not care about physical memory layout, no problem.
>>>
>>> Please correct me if i'm wrong, but my understanding was that you 
>>> wanted to use min_chunk as a way to express minimum requirements for 
>>> IOVA-contiguous memory. If i understood you correctly, i don't think 
>>> that's the way to go because there could be valid use cases where a 
>>> mempool driver would like to advertise min_chunk_size to be equal to 
>>> its total size (i.e. allocate everything in a single memzone), yet 
>>> not require that memzone to be IOVA-contiguous. I think these are 
>>> two different concerns, and one does not, and should not imply the 
>>> other.
>>
>> Aha, you're saying that virtual-contiguous and IOVA-contiguous 
>> requirements are different things that it could be usecases where 
>> virtual contiguous is important but IOVA-contiguos is not required. 
>> It is perfectly fine.
>> As I understand IOVA-contiguous (physical) typically means 
>> virtual-contiguous as well. Requirements to have everything virtually 
>> contiguous and some blocks physically contiguous are unlikely. So, it 
>> may be reduced to either virtual or physical contiguous. If mempool 
>> does not care about physical contiguous at all, 
>> MEMPOOL_F_NO_PHYS_CONTIG flag should be used and min_chunk_size 
>> should mean virtual contiguous requirements. If mempool requires 
>> physical contiguous objects, there is *no* MEMPOOL_F_NO_PHYS_CONTIG 
>> flag and min_chunk_size means physical contiguous requirements.
>>
>
> Fair point. I think we're in agreement now :) This will need to be 
> documented then.

OK, I'll do. I don't mind to rebase mine patch series on top of yours, 
but I'd like to do it a bit later when yours is closer to final version 
or even applied - it has really many prerequisites (pre-series) which 
should be collected first. It is really major changes.