Date: Wed, 9 Jan 2002 17:52:39 -0700 From: Nate Williams <nate@yogotech.com> To: Bruce Evans <bde@zeta.org.au> Cc: Nate Williams <nate@yogotech.com>, 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 Message-ID: <15420.58839.954146.527043@caddis.yogotech.com> In-Reply-To: <20020110114251.S10376-100000@gamplex.bde.org> References: <15420.34931.201187.449640@caddis.yogotech.com> <20020110114251.S10376-100000@gamplex.bde.org>
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> > > > > 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 exception? > > Only on broken systems. Why is reporting a SIGFPE considered broken? This is a valid exception, and it should be reported. I consider a system that doesn't report such exceptions broken, the opposite of what you are stating. > > > > 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. I don't have the MXX manual, just the x86/87 programming manuals for x86 (X <= 3). Nate To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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