From owner-freebsd-arch  Wed Jan  9 16:49:23 2002
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Date: Thu, 10 Jan 2002 11:49:33 +1100 (EST)
From: Bruce Evans <bde@zeta.org.au>
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To: Nate Williams <nate@yogotech.com>
Cc: Daniel Eischen <eischen@pcnet1.pcnet.com>,
	Dan Eischen <eischen@vigrid.com>, Peter Wemm <peter@wemm.org>,
	Archie Cobbs <archie@dellroad.org>, Alfred Perlstein <bright@mu.org>,
	<arch@FreeBSD.ORG>
Subject: Re: Request for review: getcontext, setcontext, etc
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On Wed, 9 Jan 2002, Nate Williams wrote:

> > > > 	fldz
> > > > 	fld1
> > > > 	fdiv %st,%st(1)		# 1.0 / 0.0, exception on this instruction
> > > > 				# but no trap yet
> > > > 	fnstsw %ax		# no trap yet since this is control instruction
> > > > 	fnclex			# forget about exception; no trap at all since
> > > > 				# we did only control instructions after the
> > > > 				# one that cause the exception
> > > >
> > > > > > It is especially wrong for fnsave in context switches because it
> > > > > > causes the signals in the context of the context switcher instead of
> > > > > > in the context of the thread that caused the exception.
> > > > >
> > > > > Which signals are you refering to?
> > > >
> > > > The SIGFPE that you get for `wait'.
> > >
> > > So, when is SIGFPE going to be triggered above, since it appears to be
> > > masked by using fnstw vs. fstw.
> >
> > When the above is interrupted by a signal, and the signal handler wants
> > to switch to another thread, and erroneously saves the context using
> > fsave instead of fnsave.
>
> Assuming I used followed things up with 'fnsave', will the exception be
> triggered by a frstor, and thus the exception will be correctly reported
> by the thread triggering the excpetion?

Only on broken systems.

> > > In other words, how do I actually make sure that I receive the
> > > exception/trapf for FP errors?
> >
> > Let the thread (appear to) continue normally by never triggering a trap
> > for the exception if you switch its context.  Using fnsave/frstor
> > accomplishes this, except on broken (old) systems where fnsave traps anyway.
>
> Is this broken hardware systems, or broken software systems?  (In other
> words, does "broken (old)" imply a bug in hardware or software?)

Mostly broken hardware.  Software can still get the broken behaviour by
using irq13 error reporting, which may still be required for backwards
compatibility.  I think W98 would have to use it for DOS mode; I don't
know about native mode.

See Appendix C of the Intel MMX manual (1997 version at least) for more
details.

Bruce


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