From owner-freebsd-arch  Thu Jan 10 11:21:30 2002
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Date: Thu, 10 Jan 2002 11:20:56 -0800
From: Terry Lambert <tlambert2@mindspring.com>
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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>,
	Alfred Perlstein <bright@mu.org>, arch@FreeBSD.ORG
Subject: Re: Request for review: getcontext, setcontext, etc
References: <20020111000813.N12236-100000@gamplex.bde.org>
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Bruce Evans 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.

How onerous would it be to look at the register in the kernel
before returning to user space, following a context switch?

I really don't think the lazy task switching FPU state saving
needs to care about threads, only about processes... and once
the bit is set, it's set, right?

-- Terry

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