linux-kernel - Re: [RFC] x86: xsave/xrstor support, ucontext

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Date:	Tue, 20 May 2008 07:58:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mikael Pettersson <mikpe@...uu.se>
CC:	Andi Kleen <andi@...stfloor.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>, mingo@...e.hu,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, roland@...hat.com, drepper@...hat.com,
	Hongjiu.lu@...el.com, linux-kernel@...r.kernel.org,
	arjan@...ux.intel.com, rmk+lkml@....linux.org.uk, dan@...ian.org,
	asit.k.mallick@...el.com
Subject: Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

Mikael Pettersson wrote:
>  > 
>  > Are they always zeroed in earlier CPUs though?  If not that wouldn't
>  > work 100% reliably because whatever cookie you put in could have been
>  > there before by chance.
> 
> I wrote a test program (fill an area with zeroes, fxsave, inspect
> reserved fields, then fill it with ones, fxsave, inspect again),
> and all processors appear to just not write anything to the reserved
> fields after the last xmm register. (Tested on an old Mobile Athlon64,
> Opteron 280, P4 Xeon, Pentium-D, and C2 Xeon E5345.)
> 
> So the question now is what if anything has the Linux kernel written
> to those reserved fields. (Looking..) Hmm, signal delivery on x86-64
> seems to do fxsave directly to the fxsave area in the user's sigframe,
> which would imply that the reserved fields have unpredictable values.
> 

OK, so that's not a usable path unless we can find some area in the 
existing data set to put a flag.  Groan.

	-hpa
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