arm64: cpufeature: add cpus_have_final_cap()

arm64: cpufeature: add cpus_have_final_cap()

When cpus_have_const_cap() was originally introduced it was intended to
be safe in hyp context, where it is not safe to access the cpu_hwcaps
array as cpus_have_cap() did. For more details see commit:

  a4023f682739439b ("arm64: Add hypervisor safe helper for checking constant capabilities")

We then made use of cpus_have_const_cap() throughout the kernel.

Subsequently, we had to defer updating the static_key associated with
each capability in order to avoid lockdep complaints. To avoid breaking
kernel-wide usage of cpus_have_const_cap(), this was updated to fall
back to the cpu_hwcaps array if called before the static_keys were
updated. As the kvm hyp code was only called later than this, the
fallback is redundant but not functionally harmful. For more details,
see commit:

  63a1e1c95e60e798 ("arm64/cpufeature: don't use mutex in bringup path")

Today we have more users of cpus_have_const_cap() which are only called
once the relevant static keys are initialized, and it would be
beneficial to avoid the redundant code.

To that end, this patch adds a new cpus_have_final_cap(), helper which
is intend to be used in code which is only run once capabilities have
been finalized, and will never check the cpus_hwcap array. This helps
the compiler to generate better code as it no longer needs to generate
code to address and test the cpus_hwcap array. To help catch misuse,
cpus_have_final_cap() will BUG() if called before capabilities are
finalized.

In hyp context, BUG() will result in a hyp panic, but the specific BUG()
instance will not be identified in the usual way.

Comments are added to the various cpus_have_*_cap() helpers to describe
the constraints on when they can be used. For clarity cpus_have_cap() is
moved above the other helpers. Similarly the helpers are updated to use
system_capabilities_finalized() consistently, and this is made
__always_inline as required by its new callers.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>