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This patch provides an option to specify a interrupt routing model
where non-secure interrupts (IRQs) are routed to EL3 instead of S-EL1.
When such an interrupt occurs, the TSPD arranges a return to
the normal world after saving any necessary context. The interrupt
routing model to route IRQs to EL3 is enabled only during STD SMC
processing. Thus the pre-emption of S-EL1 is disabled during Fast SMC
and Secure Interrupt processing.

A new build option TSPD_ROUTE_NS_INT_EL3 is introduced to change
the non secure interrupt target execution level to EL3.

Fixes ARM-software/tf-issues#225

Change-Id: Ia1e779fbbb6d627091e665c73fa6315637cfdd32

Function declarations implicitly have external linkage so do not
need the extern keyword.

Change-Id: Ia0549786796d8bf5956487e8996450a0b3d79f32

This patch introduces a framework for registering interrupts routed to
EL3. The interrupt routing model is governed by the SCR_EL3.IRQ and
FIQ bits and the security state an interrupt is generated in. The
framework recognizes three type of interrupts depending upon which
exception level and security state they should be handled in
i.e. Secure EL1 interrupts, Non-secure interrupts and EL3
interrupts. It provides an API and macros that allow a runtime service
to register an handler for a type of interrupt and specify the routing
model. The framework validates the routing model and uses the context
management framework to ensure that it is applied to the SCR_EL3 prior
to entry into the target security state. It saves the handler in
internal data structures. An API is provided to retrieve the handler
when an interrupt of a particular type is asserted. Registration is
expected to be done once by the primary CPU. The same handler and
routing model is used for all CPUs.

Support for EL3 interrupts will be added to the framework in the
future. A makefile flag has been added to allow the FVP port choose
between ARM GIC v2 and v3 support in EL3. The latter version is
currently unsupported.

A framework for handling interrupts in BL3-1 will be introduced in
subsequent patches. The default routing model in the absence of any
handlers expects no interrupts to be routed to EL3.

Change-Id: Idf7c023b34fcd4800a5980f2bef85e4b5c29e649