Message-ID: <4866541F.1060709@agner.org>
Date: Sat, 28 Jun 2008 17:09:19 +0200
From: Agner Fog <agner@agner.org>
Organization: agner@agner.org
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To: Andi Kleen <andi@firstfloor.org>
CC: Arjan van de Ven <arjan@infradead.org>, "H. Peter Anvin" <hpa@zytor.com>,
       linux-kernel@vger.kernel.org
Subject: Re: ABI change for device drivers using future AVX instruction set
References: <48626514.2040905@agner.org>	<20080625092224.736c2541@infradead.org> <4862ECAB.1040402@zytor.com>	<4864CFA5.9050901@agner.org> <20080627072231.7337ba18@infradead.org>	<4865F0DA.2050906@agner.org> <87myl5pyqo.fsf@basil.nowhere.org>
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Andi Kleen wrote:
 >If a x86 device driver doesn't use the standard kernel interfaces for 
saving/restoring extended state
 > (kernel_fpu_begin/end) it will die.That is because Linux uses lazy 
FPU saving by default and when the
 >lazy FPU exception hits kernel code it will crash because that's not 
allowed. And the standard interfaces
 >are going to handle all extended state supported by the kernel. Full 
XSAVE support should be there for
 > 2.6.27.

Thank you Andi. Finally an exact and exhaustive answer. :-)
This info is not in the "Unreliable Guide To Hacking The Linux Kernel" 
or anywhere else except deeply hidden in the archives of this mailing 
list. I had to actually look into the source code of kernel_fpu_begin to 
verify that it saves not only the FPU but also the XMM registers and 
that it disables pre-emption.

You see why I want proper documentation? If this had been documented in 
some reference that was easy to find, I wouldn't have needed to take 
your time with all these questions...

Note: Please Cc: me of answers.
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