Date: Mon, 7 Jan 2002 17:59:37 -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: <15418.17529.641960.608906@caddis.yogotech.com> In-Reply-To: <20020108111521.D2910-100000@gamplex.bde.org> References: <15418.4935.657413.312252@caddis.yogotech.com> <20020108111521.D2910-100000@gamplex.bde.org>
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> > Also, forgive my ignorance, but why are we using fnsave instead of > > fsave? From my reading of the x86 manuals, it would seem that there is > > Because we aren't broken. Go on.... > > the possibility of getting bogus FPU results with fnsave (vs. fsave) > > since by using fsave, we give the FPU a chance to finish up the current > > FPU operations before we save the state? I could see where they might > > be a race where we might end up storing intermediate results of the FPU > > in some instances. > > This is only for 8086/8087 systems, to work around a bug in the 8086. > See Morse, Isaaacson and Albert's _The 80386/387 Architechure_ p.150. > The 8086 doesn't wait for the 8087 to become idle before before > proceeding past an 8087 instruction. The synchronization must be done > in software by putting a "wait" instruction before every 8087 instruction, > even though this breaks correct operation of control instructions like > fnsave (see below). The bug was fixed on the 80286/80287. So, all FPU's post 287 do not need the 'wait' instruction? In that case, is fsave essentially the same operation as fnsave? > Manuals (up to at least the 486 one) also say that "wait" instruction > is required _after_ FPU instructions if the memory written to by the > FPU instruction is read by the CPU. E.g., the wait is required for > "fnsave mem; movl mem,%eax", and "fistpl mem; movl mem,%eax" but not > for "fnsave mem; frstor mem" or "fstpl mem; fldl mem". I've never > seen this rule followed. The manual I read had something like this, hence the reason for using fsave (vs. fnsave). > The gcc maintainers knew about it and decided > not to follow it long ago, since it would be a significant pessimization > for normal FPU code. However, it would only be an insignificant > pessimization after fnsave. So, you're saying that the manuals are wrong? > > Or, is there something else I'm missing? (The reason I ask is that the > > JDK uses fsave to be 'safe', but maybe it's not buying us anything other > > than warm fuzzies. :) > > This is negatively safe. fsave is just an abbreviation for "wait; fnsave". > Using "wait" before an FP control instruction is usually wrong because > it prevents the "no-wait" feature of FP control operations from working. > It causes any unmasked exceptions to be signaled immediately. Isn't this a good thing, since if the currently executing thread has encountered an exception, it will be signaled during the current thread's context? > 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? > It normally works OK in > -current and RELENG_4 (but not in RELENG_3) only because it has no > significant effects (all exceptions are masked by default). Nate To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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