Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

["H. Peter Anvin" <hpa@zytor.com>]

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