Document entry requirements

Currently the boot-wrapper only supports some combinations of exception
levels, with other combinations not being supported.

While we generally expect the boot-wrapper to be entered at the highest
implemented exception level, the AArch32 boot-wrapper has a comment
implying it supports being entered with something else owning EL3. As
this would require such EL3 firmware to always be in sync with the
boot-wrapper's requirements, which change over time, we don't actually
support such a configuration.

Some CPU state (such as CNTFRQ/CNTFRQ_EL0) needs to be initialized at
the highest implemented exception level, but today the boot-wrapper only
does so when entered at EL3 / Secure-PL1. Thus, today the only
completely supported configurations are EL3 / Secure-PL1, and entering
in other configurations is not entirely supported.

The aarch64 `jump_kernel` function always writes to SCTLR_EL2, which is
UNDEFINED at EL1. Hence, the aarch64 boot-wrapper does not support being
entered at EL1.

The aarch32 code assumes that any non-hyp mode is Secure PL1, and
attempt to switch to monitor mode. If entered on a system without the
security extensions, where the highest privileged mode is Non-secure
PL1, this will not work. Hence the aarch32 boot-wrapper does not support
being entered at Non-secure PL1.

Actually supporting all of these configurations requires restructuring
much of the boot-wrapper. For now, document the supported configurations
in each architecture's boot.S, and remove the misleading comment from
arch/aarch32/boot.S. Subsequent patches will improve the support and add
support for additional configurations.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
3 files changed