arm64: entry: Add fake CPU feature for unmapping the kernel at EL0

arm64: entry: Add fake CPU feature for unmapping the kernel at EL0

commit ea1e3de85e94 upstream.

Allow explicit disabling of the entry trampoline on the kernel command
line (kpti=off) by adding a fake CPU feature (ARM64_UNMAP_KERNEL_AT_EL0)
that can be used to toggle the alternative sequences in our entry code and
avoid use of the trampoline altogether if desired. This also allows us to
make use of a static key in arm64_kernel_unmapped_at_el0().

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Tested-by: Shanker Donthineni <shankerd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Alex Shi <alex.shi@linaro.org>
Conflicts:
	skip non enabled cpu features in
	arch/arm64/include/asm/cpucaps.h and
	arch/arm64/kernel/cpufeature.c
	using cpus_have_cap instead of cpus_have_const_cap in
	arch/arm64/include/asm/mmu.h