Re: [Xenomai-core] RTDM fd support.

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>> Gilles Chanteperdrix wrote:
>>>> Jan Kiszka wrote:
>>>>> Gilles Chanteperdrix wrote:
>>>>>> Hi Jan,
>>>>>>
>>>>>> from discussions on the mailing list, it seems that we are going to need
>>>>>> that unified file descriptors thing. However, since everybody wants
>>>>>> 2.5.0 to be released ASAP, we should try to think about any changes for
>>>>>> this support which would break the ABI, do them now, and keep the rest
>>>>>> for later.
>>>>>>
>>>>>> One such problem is the translation which currently exists between rtdm
>>>>>> file descriptors and descriptors passed to the posix skin, by adding
>>>>>> 1024 - 128. So, I propose to fix this issue.
>>>>>>
>>>>>> The idea Philippe had, and which I tend to agree to, was, in case of
>>>>>> open/socket/accept, to open("/dev/null"), and use an association table
>>>>>> somewhere to associate with the kernel-space descriptor number. Since
>>>>>> we are at it, this association table could in fact be the file
>>>>>> descriptor table, which we would put in the core skin ppd. The actual
>>>>>> data structure should be sparse, a linked list does not scale, so,
>>>>>> probably a hash would do. (I could also propose a solution based on avl
>>>>>> trees, but their implementation is not nearly as simple).
>>>>>>
>>>>>> Additionnally, this would allow our open/socket to conform to posix
>>>>>> which states that open should return the lowest free file descriptor.
>>>>>>
>>>>>> Should I propose a patch in that direction? Do you see any other
>>>>>> possible cause of ABI breakage when we migrate to an unified file
>>>>>> descriptors structure?
>>>>> Right now this sounds like a plan - but I don't feel 100% comfortable to
>>>>> predict that we will get along with it. Converting some skin-specific
>>>>> service to a generic one involves quite a lot of refactoring. It is not
>>>>> really unlikely that we define some ABI now that will later turn out to
>>>>> be insufficient for what we want to achieve.
>>>> For the posix skin, I can live with a two hops solution right now, and
>>>> implement the one-hop solution later.
>>>>
>>>> user-space fd
>>>>       |
>>>>       | core skin fd table
>>>>       v
>>>> kernel-space fd
>>>>       |
>>>>       | posix skin registry
>>>>       v
>>>> actual object
>>>>
>>>> If we do this, there is not much re-factoring involved.
>>>>
>>>> The question really boils down to whether you accept this solution for
>>>> the rtdm skin too.
>>> Let me check if I got the idea: the first change would only modify the
>>> librtdm and the rtdm part of libpthread_rt, right?
>> The idea was to also modify the kernel-space skins. Only make it
>> superficial: only modify the part which communicates the file
>> descriptors to user-space, to include a lookup through the fd table.
>>
>> So, this means that when a user-space applications calls read() for
>> instance, the fd table is used to get the kernel-space RTDM descriptor,
>> and then the internal functions of the RTDM skin, go through their own
>> lookup mechanim to get the actual object.
>>
>> So, there are two lookups, that is the two hops I was talking about. I
>> was worried that you would not like the impact on performance.
> 
> Yes, I'm a bit. And I do not get the significant advantage of this
> approach over the final one-hop approach anymore.

More superficial modifications, we do not break the posix and rtdm skins
internal registries.

-- 
					    Gilles.