Re: [RFC PATCH 0/6] Xenomai: Real-time Exception Handling

[Richard Weinberger <richard@sigma-star.at>]

On Dienstag, 8. April 2025 19:32 Jan Kiszka wrote:
> > real-time.  A notable example is a page fault, determining whether
> > SIGSEGV is appropriate requires deep traversal into the memory
> > management code.
> 
> ...on arm and arm64. It worked fairly well for x86. This is indeed a
> very prominent exception that people may expect behind this feature. And
> it is also a use case we have with our (x86-only) deployment.

True that.
I think the set of supported exceptions should be architecture specific
anyways. And we need to state that it is best effort.

> 
> > 
> > Patches 1, 2, 3, and 4 implement the Xenomai-side changes for this
> > feature.  Dovetail will receive a separate patch set. Patches 5 and 6
> > introduce tests.
> > 
> > It is important to note that this is not a fully-fledged signal
> > implementation.  The only supported use case is delivering exceptions
> > as signals to the affected thread.
> > There is no support for sending signals or handling block/ignore masks.
> > The only user visible API so far is cobalt_rt_signal().
> > It takes a signal number (SIGILL or SIGFPE so far) and a handler
> > function with the signature fn(int, siginfo_t *, void *).
> 
> Did you intentionally left out the oldact parameter of sigaction?

Not really. Adding this should be trivial.
Maybe my subconsciousness tried to make it less look like POSIX
real time signals. ;-)

> 
> > 
> > TODO:
> > - Better naming, IMHO "signal" is the wrong term and will confuse users.
> >   Especially since POSIX real-time signals are something different.
> >   Maybe "umex" for "user mode exception handling"?
> 
> Just "exception" might be enough and clearer. OTOH, we are reusing a lot
> of sigaction and siginfo...

That's the problem. It kinda looks like signals but isn't.

> 
> > - Document new APIs
> > - Improve tests
> > - Carefully select more exceptions
> 
> Yeah, that is the challenge.
> 
> And:
> 
>  - align with EVL / Xenomai 4 to ensure compatible features

Agreed.
The dovetail side is rather small. We could also integrate the signal frame
helper functions into Xenomai.

Thanks,
//richard

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