Re: [Xenomai-help] Exception handlers in primary domain / user-space signals

[Philippe Gerum <rpm@xenomai.org>]

On Tue, 2011-02-08 at 13:51 +0100, Henri Roosen wrote:
> On Tue, Feb 8, 2011 at 1:31 PM, Gilles Chanteperdrix
> <gilles.chanteperdrix@xenomai.org> wrote:
> > Philippe Gerum wrote:
> >> On Tue, 2011-02-08 at 13:16 +0100, Gilles Chanteperdrix wrote:
> >>> I was not talking about the Xenomai case specifically, but since Henri
> >>> would like to have the full signals implementation with Xenomai, this
> >>> does a apply to Xenomai too.
> >>>
> >>>
> >>
> >> I think we all agree that having a complete signal implementation for
> >> Xenomai in pure rt mode won't happen overnight. So the point is now: how
> >> could it be mimicked, at least for the most useful part.
> >>
> >
> > My point is that whatever you do, a switch user-kernel, then kernel-user
> > is not going to be lightweight, so avoiding it in the application in the
> > first place may be a better idea.
> >
> > My aim with implementing complete signals was rather for things like
> > timer_* and mq_notify, where the interface requires them, I did not even
> > imagine implementing SIGFPE, SIGILL, SIGTRAP, which I thought could not
> > be time critical anyway, for the reasons explained earlier. So, my
> > question (rather to Henri) is: what would we need SIGFPE, SIGILL,
> > SIGTRAP in an real-time application for?
> 
> I agree it might be unusual. For the tracing use case: the SIGTRAP we
> use as a means for tracing whether code is actually executed, just
> like breakpoints, we exchange the code to 0xcc and handle the
> exceptions do book-keeping but don't stop the task. We know this has
> overhead, it also had when using our old OS. The old OS handled it in
> an accepted amount of time. Using the Xenomai kernel it also works,
> however the overhead is not acceptable anymore.
> Installing a floating point exception handler was also provided to our
> customers with the old OS and we have to make that available now too.
> So actually it is all because of legacy reasons, we have to provide
> similar functionality as with the old OS.
> 
> I'm afraid we cannot mimic enough so it suits our use cases. We need
> the fault context to handle the exception and to set the IP one
> instruction back.

So you need the signal rebase over the mayday support I merged a few
months ago. Back to square one I'm afraid, this won't be available soon,
albeit this might happen in the 2.6 timeframe. We'll see.

> 
> Thanks,
> Henri.
> 
> >
> > --
> >                                                                Gilles.
> >

-- 
Philippe.