powerpc: Don't corrupt user registers on 32-bit

[Scott Wood]

On Wed, Oct 23, 2013 at 09:40:02AM +0100, Paul Mackerras wrote:
> Commit de79f7b9f6 ("powerpc: Put FP/VSX and VR state into structures")
> modified load_up_fpu() and load_up_altivec() in such a way that they
> now use r7 and r8.  Unfortunately, the callers of these functions on
> 32-bit machines then return to userspace via fast_exception_return,
> which doesn't restore all of the volatile GPRs, but only r1, r3 -- r6
> and r9 -- r12.  This was causing userspace segfaults and other
> userspace misbehaviour on 32-bit machines.
> 
> This fixes the problem by changing the register usage of load_up_fpu()
> and load_up_altivec() to avoid using r7 and r8 and instead use r6 and
> r10.  This also adds comments to those functions saying which registers
> may be used.
> 
> Signed-off-by: Paul Mackerras <paulus at samba.org>
> 
> ---
> arch/powerpc/kernel/fpu.S    | 14 ++++++++------
>  arch/powerpc/kernel/vector.S | 15 +++++++++------
>  2 files changed, 17 insertions(+), 12 deletions(-)

Tested-by: Scott Wood <scottwood at freescale.com> (on e500mc, so no altivec)

-Scott