Date: Mon, 7 Jan 2002 13:18:31 -0700 From: Nate Williams <nate@yogotech.com> To: Alfred Perlstein <bright@mu.org> Cc: Nate Williams <nate@yogotech.com>, Dan Eischen <eischen@vigrid.com>, Peter Wemm <peter@wemm.org>, Archie Cobbs <archie@dellroad.org>, arch@FreeBSD.ORG Subject: Re: Request for review: getcontext, setcontext, etc Message-ID: <15418.663.130281.835301@caddis.yogotech.com> In-Reply-To: <20020107130208.E18706@elvis.mu.org> References: <20020106232937.9F87D38CC@overcee.netplex.com.au> <3C390746.5FE7648C@vigrid.com> <15417.59947.662052.836634@caddis.yogotech.com> <20020107130208.E18706@elvis.mu.org>
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> > > > Also, I noticed that the i386 patch doesn't save FP state (!) which is > > > > one of the primary reasons for get/setcontext(). I'm not sure if this > > > > can be efficiently done since this user-level function will not know if > > > > the current context has touched the FPU yet.. > > > > > > Neither does the kernel, does it? I thought I saw comments in the > > > kernel (was it alpha?) about it being too bad that we couldn't tell > > > if the FPU was used. In libc_r, we currently only save and restore the > > > FP state when the context is generated from a signal handler (or perhaps > > > in the case of KSEs, when the thread was preempted). > > > > Hmm, IIRC, Java's green threads saves the FP context everytime it does a > > thread switch, since it has no way of knowing if the thread was doing FP > > context. Is there a way to force get/setcontext to always/conditionally > > save the FP context, for applications that either know they need to have > > it saved? > > Well, one trick might be to do this: > > 1) Request that the OS turn off the FP enabled bit when running new > threads. In userland threads, there is no way to turn it off on a thread basis, so it's either on/off for the entire process. > 2) If you get an FP not present exception you mark that thread as > using FP. At this point, we'd have to turn it on for all threads, since any future thread that uses the context would also need to save/restore the thread. (Since, FPU would be turned on for this 'process'.) > 3) In the threads scheduler and at context switch time between it (the > thread) and any other thread not using FP you request the behavior > of point #1. > > So you would only have a syscall when switching between FP and non-FP > threads. > > Is this possible? In the future it may be possible, if we keep track of FP-usage on a thread basis instead of a process basis. This requires kernel threads. Nate To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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