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

[Philippe Gerum <rpm@xenomai.org>]

On Tue, 2011-02-08 at 14:11 +0100, Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
> > 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.
> 
> Well, not necessarily, the fault handler may set the IP, suspend the
> task, wake-up the dedicated fault-handler thread, then, the dedicated
> fault-handler may wake-up the suspended task when the work has been done.
> 

This is not exactly what I'd call a straightforward solution (which was
the point of offloading the work to userland) . If one knows how to do
that within the Xenomai core, he could just re-route the mayday
mechanism, no need for sideways.

-- 
Philippe.