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

[Mikael Pettersson]

H. Peter Anvin writes:
 > 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.

An ugly workaround could be to start clearing one of these fields,
and say that the data there is only valid for kernels >= 2.6.26.
(I said it was ugly...)

Or we go back to stashing a flag in uc_flags (which is kosher),
and try to figure out how to mark non-rt sigframes.

/Mikael
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