arm64: Avoid cpus_have_const_cap() for ARM64_HAS_EPAN

arm64: Avoid cpus_have_const_cap() for ARM64_HAS_EPAN

We use cpus_have_const_cap() to check for ARM64_HAS_EPAN but this is not
necessary and alternative_has_cap() or cpus_have_cap() would be
preferable.

For historical reasons, cpus_have_const_cap() is more complicated than
it needs to be. Before cpucaps are finalized, it will perform a bitmap
test of the system_cpucaps bitmap, and once cpucaps are finalized it
will use an alternative branch. This used to be necessary to handle some
race conditions in the window between cpucap detection and the
subsequent patching of alternatives and static branches, where different
branches could be out-of-sync with one another (or w.r.t. alternative
sequences). Now that we use alternative branches instead of static
branches, these are all patched atomically w.r.t. one another, and there
are only a handful of cases that need special care in the window between
cpucap detection and alternative patching.

Due to the above, it would be nice to remove cpus_have_const_cap(), and
migrate callers over to alternative_has_cap_*(), cpus_have_final_cap(),
or cpus_have_cap() depending on when their requirements. This will
remove redundant instructions and improve code generation, and will make
it easier to determine how each callsite will behave before, during, and
after alternative patching.

The ARM64_HAS_EPAN cpucap is used to affect two things:

1) The permision bits used for userspace executable mappings, which are
   chosen by adjust_protection_map(), which is an arch_initcall. This is
   called after the ARM64_HAS_EPAN cpucap has been detected and
   alternatives have been patched, and before any userspace translation
   tables exist.

2) The handling of faults taken from (user or kernel) accesses to
   userspace executable mappings in do_page_fault(). Userspace
   translation tables are created after adjust_protection_map() is
   called, and hence after the ARM64_HAS_EPAN cpucap has been detected
   and alternatives have been patched.

Neither of these run until after ARM64_HAS_EPAN cpucap has been detected
and alternatives have been patched, and hence there's no need to use
cpus_have_const_cap(). Since adjust_protection_map() is only executed
once at boot time it would be best for it to use cpus_have_cap(), and
since do_page_fault() is executed frequently it would be best for it to
use alternatives_have_cap_unlikely().

This patch replaces the uses of cpus_have_const_cap() with
cpus_have_cap() and alternative_has_cap_unlikely(), which will avoid
generating redundant code, and should be better for all subsequent calls
at runtime. The ARM64_HAS_EPAN cpucap is added to cpucap_is_possible()
so that code can be elided entirely when this is not possible.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Vladimir Murzin <vladimir.murzin@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>