powerpc: Provide a way for KVM to indicate that NV GPR values are lost

powerpc: Provide a way for KVM to indicate that NV GPR values are lost

This fixes a problem where a CPU thread coming out of nap mode can
think it has valid values in the nonvolatile GPRs (r14 - r31) as saved
away in power7_idle, but in fact the values have been trashed because
the thread was used for KVM in the mean time.  The result is that the
thread crashes because code that called power7_idle (e.g.,
pnv_smp_cpu_kill_self()) goes to use values in registers that have
been trashed.

The bit field in SRR1 that tells whether state was lost only reflects
the most recent nap, which may not have been the nap instruction in
power7_idle.  So we need an extra PACA field to indicate that state
has been lost even if SRR1 indicates that the most recent nap didn't
lose state.  We clear this field when saving the state in power7_idle,
we set it to a non-zero value when we use the thread for KVM, and we
test it in power7_wakeup_noloss.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>