arm64/head: Disable MMU at EL2 before clearing HCR_EL2.E2H

arm64/head: Disable MMU at EL2 before clearing HCR_EL2.E2H

Even though the boot protocol stipulates otherwise, an exception has
been made for the EFI stub, and entering the core kernel with the MMU
enabled is permitted. This allows a substantial amount of cache
maintenance to be elided, wich is significant when fast boot times are
critical (e.g., for booting micro-VMs)

Once the initial ID map has been populated, the MMU is disabled as part
of the logic sequence that puts all system registers into a known state.
Any code that needs to execute within the window where the MMU is off is
cleaned to the PoC explicitly, which includes all of HYP text when
entering at EL2.

However, the current sequence of initializing the EL2 system registers
is not safe: HCR_EL2 is set to its nVHE initial state before SCTLR_EL2
is reprogrammed, and this means that a VHE-to-nVHE switch may occur
while the MMU is enabled. This switch causes some system registers as
well as page table descriptors to be interpreted in a different way,
potentially resulting in spurious exceptions relating to MMU
translation.

So disable the MMU explicitly first when entering in EL2 with the MMU
and caches enabled.

Fixes: 617861703830 ("efi: arm64: enter with MMU and caches enabled")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: <stable@vger.kernel.org> # 6.3.x
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240415075412.2347624-6-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>