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

From: Mikael Pettersson
Date: Tue May 20 2008 - 11:24:23 EST

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.

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