From owner-freebsd-arch  Thu Jan 10  8:55:41 2002
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Date: Thu, 10 Jan 2002 08:55:38 -0800
From: Alfred Perlstein <bright@mu.org>
To: Bruce Evans <bde@zeta.org.au>
Cc: Peter Wemm <peter@wemm.org>, Nate Williams <nate@yogotech.com>,
	Dan Eischen <eischen@vigrid.com>, Archie Cobbs <archie@dellroad.org>,
	arch@FreeBSD.ORG
Subject: Re: Request for review: getcontext, setcontext, etc
Message-ID: <20020110085538.A7984@elvis.mu.org>
References: <20020110090055.B28BC38CC@overcee.netplex.com.au> <20020111000813.N12236-100000@gamplex.bde.org>
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* Bruce Evans <bde@zeta.org.au> [020110 05:14] wrote:
> On Thu, 10 Jan 2002, Peter Wemm wrote:
> 
> > Nate Williams wrote:
> > > 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?
> >
> > Exactly the problem.  Userland cannot tell if it has touched FP or not
> > except by touching it.  This is where a system call is more efficient since
> > to save FP context you are doing trap recovery on top of doing the work.
> 
> Actually, userland can look at the state bit in %msw (on i386's) to
> tell if it can look at the state without trapping.  However, this is
> not very useful.  The state will often be in the kernel, and then
> userland will have to trap to the kernel too look at it.  The trap
> may as well be a syscall that fetches all the state.

The userland code might request that a special word be written to
in user memory when the FP trap is taken.

int fpnotify(int *flag);

/* scheduling... */
syscall_fpnotify(&fpused);
/* run thread */
/* back to scheduler */
if (fpused) {  /* kernel has suword'd that the fp was dirtied */
   syscall_fpclear_and_notify(&fpused); /* reset state */

} else {       /* fp wasn't touched */


}

The idea being to only require the syscall to reset it if it's 
used and you are switching between fp and non-fp using threads.

-Alfred

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