Wolfgang Mauerer wrote:
> Gilles Chanteperdrix wrote:
>> Wolfgang Mauerer wrote:
>>>>> +enum xnshared_features {
>>>>> +/*       XNSHARED_FEAT_A = 1,
>>>>> + XNSHARED_FEAT_B, */
>>>>> + XNSHARED_MAX_FEATURES,
>>>>> +};
>>>> Xenomai style is to use defines bit with the bit shifted directly, so,
>>>> XNSHARED_FEAT_A would rather be 0x0000000000000001, and I am not sure an
>>>> enum can be 64 bits.
>>> oh yeah, enums are always ints, as a superficial look into the
>>> ANSI standard tells me. Wasn't thinking about that. Using
>>> your direct definition makes sense (although it's a bit
>>> unlikely that there will ever be more than 2^32 shared
>>> features...)
>> Err, since features are bits, the limit is 32 features, not 2^32, and if
>> we make the features member an unsigned long long in the first place, it
>> is because we expect to be able to use the 64 bits, no?
> 
> Argl. Yes. /me realises that christmas holidays are severely required...
>>>> Ok. There is a real question here. Whether we want to put this code in
>>>> module.c or shadow.c. I have no definite answer. Arguments for each file:
>>>>
>>>> module.c: the shared area will be used for NTP by the clock/timer
>>>> subsystem, so will be used in kernel-space, even without
>>>> CONFIG_XENO_OPT_PERVASIVE.
>>>>
>>>> shadow.c: global shared heap is uncached on ARM, so, I am not sure we
>>>> really want the timer/clock system to use the same area as user-space.
>>>> So, we are probably better off using different copies of the same data
>>>> in kernel-space and user-space (with a price of a copy every time the
>>>> data change and CONFIG_XENO_OPT_PERVASIVE is enabled). This would allow
>>>> us to move that code in shadow.c
>>> Is there any compelling advantage for shadow.c? Using a copy operation
>>> every time the data change seems a bit contrary to the reason for
>>> the new mechanism in the first place, namely to have a light-weight
>>> means of sharing data.
>> The way I see it, the NTP data will be used by the nucleus clock and
>> timer subsystem, so several times for each timer tick, so having them on
>> an uncached memory will have a performance hit.
>>
>> The copy, on the other hand, would take place over a non real-time
>> context, only once every update of NTP data, i.e. once every 10ms.
> 
>> So, let us move it to shadow.c.
> 
> I see your point, but it has the disadvantage that the time spent
> in the Linux kernel for the copy operations in the vsyscall is
> increased -- which might affect the Linux domain, since this is
> an optimised hot path. Naturally, in the absence of any quantitative
> measurements, this is all just speculation, but if access to the global
> semaphore heap is unchached only on ARM, we provide an unnecessary
> optimisation for n-1 out of n architectures supported by Xenomai.
> 
> Having the time-keeping code in the Xenomai kernel benefit from NTP
> corrections provided by the Linux side is a bit separate from the
> ABI change, which would be the same irregardless of whether we
> use a second copy for kernel-side NTP operations or not.

Exactly, the issue are distinct, the ABI change is a pure user-space
issue, so, should only be compiled if CONFIG_XENO_OPT_PERVASIVE is defined.

 Would it
> therefore make sense to restrict the data exchange to the global heap
> semaphore, and add an architecture-specific hook for ARM
> later on to generate an extra kernel space copy of the NTP data? The
> Linux vsyscall time keeping information is architecture-specific, so

No. We will make it generic. Nothing arch specific is needed. We will
not copy the vsyscall time keeping information, we will copy the data
passed to update_vsyscall, which are the same on all architectures.

> arch specific hooks make sense in this area anyway. We could therefore
> restrict the second copy to ARM without much effort. Or am I missing
> something in your considerations?

This xnvdso feature is needed for user-space only. Let us move it to
shadow.c.

>> The really really nice thing to do, would be a unit test which checks
>> for a specific value of the ABI features. So, we can run
>> "check_xeno_abi_features 0", to check that the thing actually works with
>> release 2.5.0.
> 
> okay, so I'll make sure to have the userland bits ready in time
> before the 2.5.0 release.

You can implement the user-space part only in a unit test, if it is
easier. All that I ask, is to be able to test that it works.

> 
> Cheers, Wolfgang


-- 
                                            Gilles.

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