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

[Suresh Siddha <suresh.b.siddha@intel.com>]

On Thu, May 22, 2008 at 05:33:30PM -0700, H. Peter Anvin wrote:
> Mikael Pettersson wrote:
> >
> > > > While restoring from the user, kernel also need to find out what 
> > layout
> > > > the user is passing. So it's bi-directional. I prefer the same 
> > mechanism
> > > > (using cookies/magic numbers etc inaddition to uc_flags or cpuid 
> > checks) to
> > > > interpret the fpstate for both user/kernel.
> > > 
> > > No, it really doesn't: the kernel only needs to be able to read the 
> > same > format as it itself wrote.
> >
> >The kernel needs to accept one(*) of the formats it can produce, which
> >is not necessarily what it last produced. It's not inconceivable that
> >user-space will construct sigframes on the fly (to emulate setcontext),
> >or that it will mangle sigframes (e.g. to map non-rt to rt before 
> >sigreturn).
> >
> >(*) The format is determined by which version of sys_sigreturn the
> >user invokes.
> >
> 
> No.  You CANNOT restore from a frame that doesn't have the full state - 
> you don't have enough information to do so!

What I was doing in the RFC is: restore the state what ever that was present and
init the state that was not present in the stack frame.