Re: [Xenomai-core] [PATCH] Use tsc for implementation of clock_gettime.

[Jan Kiszka <jan.kiszka@domain.hid>]

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Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>  > Gilles Chanteperdrix wrote:
>  > > Hi,
>  > > 
>  > > here comes, for review, a patch which reduces the overhead of
>  > > clock_gettime by directly reading the tsc in user-space for
>  > > architectures that support it.
>  > 
>  > Highly welcome. But I have one concern: How and when do you propagate
>  > wallclock_offset changes to user space?
> 
> Since clock_settime is not implemented, never, but if clock_settime was
> implemented, clock_settime would re-issue the __xn_sys_info syscall.

This excludes automatic clock adjustment, something I'm convinced we
will have to provide in the future.

> 
>  > 
>  > I think we need some vsyscall-alike approach for this, some read-only
>  > page that is mapped into every RT process, containing things like a
>  > regularly updated offset (seqlock fashioned) or other read-only
>  > information (shadow mode? cpu id?).
> 
> As I said yesterday, having a page mapped at a different address in
> kernel-space and user-space is not an option on ARM, because each
> mapping of this page would have a separated cache.
> 

Hmm, what about the kernel writing directly to the (current) user
address? We then only need to keep track of the per-mm address and have
not fixed kernel equivalent. At least on arm, other archs may handle
this differently at compile time.

Jan


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