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Date: Mon, 10 Mar 2003 14:59:53 -0500
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From: Chris Friesen <cfriesen@nortelnetworks.com>
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To: Linus Torvalds <torvalds@transmeta.com>
Cc: "David S. Miller" <davem@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [BK-2.5] Move "used FPU status" into new non-atomic thread_info->status field.
References: <Pine.LNX.4.44.0303101119220.2240-100000@home.transmeta.com>
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Linus Torvalds wrote:
> On Mon, 10 Mar 2003, David S. Miller wrote:
> 
>>   
>>At least on sparc{32,64}, we consider FPU state to be clobbered coming
>>into system calls, this eliminates a lot of hair wrt. FPU state
>>restoring in cases such as fork().
>>
> 
> We could _probably_ do it on x86 too. The standard C calling convention on 
> x86 says FPU register state is clobbered, if I remember correctly. 
> However, some of the state is "long-term", like rounding modes, exception 
> masking etc, and even if we didn't save the register state we would have 
> to save that part.
> 
> And once you save that part, you're better off saving the registers too, 
> since it's all loaded and saved with the same fxsave/fxrestor instruction 
> (ie we'd actually have to do _more_ work to save only part of the FP 
> state).

Does this open the door for using FP in the kernel?

Chris


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