linux-kernel - Re: [PATCH 2/2] i387: split up <asm/i387.h> into exported and internal interfaces

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Date:	Tue, 28 Feb 2012 10:08:44 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Josh Boyer <jwboyer@...il.com>,
	Jongman Heo <jongman.heo@...sung.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/2] i387: split up <asm/i387.h> into exported and
 internal interfaces

On Tue, Feb 28, 2012 at 9:37 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> So where's the comment about why you actually own and control CR0.TS,
> and nobody else does?

So what I think KVM should strive for (but I really don't know the
code, so maybe there are good reasons why it is impossible) is to just
never touch TS at all, and let the core kernel code do it all for you.

When you need access to the FPU, let the core code just handle it for
you. Let it trap and restore the state. When you get scheduled away,
let the core code just set TS, because you really can't touch the FP
state again.

IOW, just do the FP operations you do within the thread you are. Never
touch TS at all, just don't worry about it. Worry about your own
internal FP state machine, but don't interact with the "global" kernel
TS state machine.

You can't do a lot better than that, I think. Especially now that we
do the lazy restore, we can schedule between two tasks and if only one
of them actually uses the FPU, we won't bother with extraneous state
restores.

The one exception I can think of is that if you are loading totally
*new* FP state, and you think that TS is likely to be set, instead of
trapping (and loading the old state in the trap handling) only to
return to load the *new* state, we could expose a helper for that
situation. It would look something like

   user_fpu_begin();
   fpu_restore_checking(newfpustate);

and it would avoid the trap when loading the new state.

                       Linus
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