arm64: Require that system registers at all visible ELs be initialized

Currently we require that software at a higher exception level initialise
all registers at the exception level the kernel will be entered prior to
starting the kernel in order to ensure that there is nothing uninitialised
which could result in an UNKNOWN state while running the kernel. The
expectation is that the software running at the highest exception levels
will be tightly coupled to the system and can ensure that all available
features are appropriately initialised and that the kernel can initialise
anything else.

There is a gap here in the case where new registers are added to lower
exception levels that require initialisation but the kernel does not yet
understand them. Extend the requirement to also include exception levels
below the one where the kernel is entered to cover this.

Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210401180942.35815-4-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/booting.rst

@@ -202,9 +202,10 @@ Before jumping into the kernel, the following conditions must be met:
 
 - System registers
 
-  All writable architected system registers at the exception level where
-  the kernel image will be entered must be initialised by software at a
-  higher exception level to prevent execution in an UNKNOWN state.
+  All writable architected system registers at or below the exception
+  level where the kernel image will be entered must be initialised by
+  software at a higher exception level to prevent execution in an UNKNOWN
+  state.
 
   - SCR_EL3.FIQ must have the same value across all CPUs the kernel is
     executing on.