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Date: Mon, 14 Jan 2002 08:53:30 +1100
From: Peter Jeremy <peter.jeremy@alcatel.com.au>
Subject: Re: Request for review: getcontext, setcontext, etc
In-reply-to: <20020110135217.M7984@elvis.mu.org>; from bright@mu.org on Thu,
 Jan 10, 2002 at 01:52:17PM -0800
To: Alfred Perlstein <bright@mu.org>
Cc: Kelly Yancey <kbyanc@posi.net>,
	Nate Williams <nate@yogotech.com>,
	Terry Lambert <tlambert2@mindspring.com>,
	Daniel Eischen <eischen@pcnet1.pcnet.com>,
	Dan Eischen <eischen@vigrid.com>, Peter Wemm <peter@wemm.org>,
	Archie Cobbs <archie@dellroad.org>, arch@FreeBSD.ORG
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On Thu, Jan 10, 2002 at 01:52:17PM -0800, Alfred Perlstein wrote:
>1) Is atomicity required?  (looks like a "no")
>2) Are states like FP usage trackable from userspace?
>   (looks like a "yes" with some kernel help)

I gather you are thinking of a userland approach like:

	if (thread_has_used_FP) {
		save_FP_context(curthread);
		mark_FP_unused();
	}
	save_nonFP_context(curthread);
	restore_nonFP_context(newthread);
	if (FP_is_used(newthread))
		pass_FP_context_to_kernel(newthread);
	curthread = newthread;

Since FP status is per-CPU, this needs atomicity to an
extent:  You must ensure that the active thread does not
migrate between CPUs between the "thread_has_used_FP" test
and following save.  I agree this is unlikely, but it needs
to be catered for.

Peter

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