spapr: Don't trigger a CAS reboot for XICS/XIVE mode changeover

spapr: Don't trigger a CAS reboot for XICS/XIVE mode changeover

PAPR allows the interrupt controller used on a POWER9 machine (XICS or
XIVE) to be selected by the guest operating system, by using the
ibm,client-architecture-support (CAS) feature negotiation call.

Currently, if the guest selects an interrupt controller different from the
one selected at initial boot, this causes the system to be reset with the
new model and the boot starts again.  This means we run through the SLOF
boot process twice, as well as any other bootloader (e.g. grub) in use
before the OS calls CAS.  This can be confusing and/or inconvenient for
users.

Thanks to two fairly recent changes, we no longer need this reboot.  1) we
now completely regenerate the device tree when CAS is called (meaning we
don't need special case updates for all the device tree changes caused by
the interrupt controller mode change),  2) we now have explicit code paths
to activate and deactivate the different interrupt controllers, rather than
just implicitly calling those at machine reset time.

We can therefore eliminate the reboot for changing irq mode, simply by
putting a call to spapr_irq_update_active_intc() before we call
spapr_h_cas_compose_response() (which gives the updated device tree to
the guest firmware and OS).

Signed-off-by: David Gibson <[email protected]>
Reviewed-by: Cedric Le Goater <[email protected]>
Reviewed-by: Greg Kurz <[email protected]>