arm64: Delay enabling hardware DBM feature

We enable hardware DBM bit in a capable CPU, very early in the
boot via __cpu_setup. This doesn't give us a flexibility of
optionally disable the feature, as the clearing the bit
is a bit costly as the TLB can cache the settings. Instead,
we delay enabling the feature until the CPU is brought up
into the kernel. We use the feature capability mechanism
to handle it.

The hardware DBM is a non-conflicting feature. i.e, the kernel
can safely run with a mix of CPUs with some using the feature
and the others don't. So, it is safe for a late CPU to have
this capability and enable it, even if the active CPUs don't.

To get this handled properly by the infrastructure, we
unconditionally set the capability and only enable it
on CPUs which really have the feature. Also, we print the
feature detection from the "matches" call back to make sure
we don't mislead the user when none of the CPUs could use the
feature.

Change-Id: Icb987abe0a4d3eeae718c88e53fc9a8f3cce7f80
Cc: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Dave Martin <dave.martin@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: 05abb595bbaccc9c4290bee62086d0eeea9f0f32
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: remove code that is not needed for
 having a way of expressing the list of CPUs that would
 be impacted by erratum 1024718. Add calls to
 preemption_disable and preemption_enable to conform to
 the proper use of has_cpuid_feature]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>