Message-ID: <486807D6.9060407@zytor.com>
Date: Sun, 29 Jun 2008 15:08:22 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andi Kleen <andi@firstfloor.org>
CC: Avi Kivity <avi@qumranet.com>, Agner Fog <agner@agner.org>,
       Arjan van de Ven <arjan@infradead.org>, 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> <4866541F.1060709@agner.org> <20080628200231.GA21259@one.firstfloor.org> <48677313.8000804@qumranet.com> <48677E39.6000001@firstfloor.org> <486780BA.6010202@qumranet.com> <486788F4.5060202@firstfloor.org> <48678BBF.80707@qumranet.com> <48678CC5.5080504@firstfloor.org>
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Andi Kleen wrote:
>>>   
>> If you use xsave, I don't see how this is different to the user fpu save
>> area.
> 
> For once there's no clear error handling path for allocation failures
> on the (arbitarily sized) xsave state. On user code that can be barely
> tolerated, but for the kernel it would be deadly.
> 

Well, that's relatively easily dealt with... you'd have to allocate that 
state save area explicitly in kernel_fpu_begin(), and it would be up to 
the callers of that function to handle the resulting sleep and/or 
allocation failure -- we could even make kernel_fpu_begin() take a GFP_* 
flag.

Now, there are a few possibilities what to do with said state area.  One 
is to make it associated with the task; if used for something as RAID, 
this would mean pretty soon *all* (or nearly all) tasks have such a 
state area, and we might as well go back to allocating them preemptively 
on thread creation.  The other, of course, is to destroy it in 
kernel_fpu_end().  This may cause quite a bit of allocation/deallocation 
overhead, but perhaps this would be a decent use of a quicklist.

	-hpa


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