xen/arm: Setup virtual paging for non-boot CPUs on hotplug/resume

xen/arm: Setup virtual paging for non-boot CPUs on hotplug/resume

In existing code the virtual paging for non-boot CPUs is setup only on boot.
The setup is triggered from start_xen() after all CPUs are brought online.
In other words, the initialization of VTCR_EL2 register is done out of the
cpu_up/start_secondary() control flow. However, the cpu_up flow is also used
to hotplug non-boot CPUs on resume from suspend to RAM state, in which case
the virtual paging will not be configured.

With this patch the setting of paging is triggered from start_secondary()
function using cpu starting notifier (notify_cpu_starting() call). The
notifier is registered in p2m.c using init call. This has to be done with
init call rather than presmp_init because the registered callback depends
on vtcr configuration value which is setup after the presmp init calls
are executed (do_presmp_initcalls() called from start_xen()). Init calls
are executed after initial virtual paging is set up for all CPUs on boot.
This ensures that no callback can fire until the vtcr value is calculated
by Xen and virtual paging is set up initially for all CPUs. Also, this way
the virtual paging setup in boot scenario remains unchanged.

It is assumed here that after the system completed the boot, CPUs that
execute start_secondary() were booted as well when the Xen itself was
booted. According to this assumption non-boot CPUs will always be compliant
with the VTCR_EL2 value that was selected by Xen on boot.
Currently, there is no mechanism to trigger hotplugging of a CPU. This
will be added with the suspend to RAM support for ARM, where the hotplug
of non-boot CPUs will be triggered via enable_nonboot_cpus() call.

Signed-off-by: Mirela Simonovic <mirela.simonovic@aggios.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>