arm64: jump labels: NOP out NOP -> NOP replacement

arm64: jump labels: NOP out NOP -> NOP replacement

In the arm64 arch_static_branch implementation we place an A64 NOP into
the instruction stream and log relevant details to a jump_entry in a
__jump_table section. Later this may be replaced with an immediate
branch without link to the code for the unlikely case.

At init time, the core calls arch_jump_label_transform_static to
initialise the NOPs. On x86 this involves inserting the optimal NOP for
a given microarchitecture, but on arm64 we only use the architectural
NOP, and hence replace each NOP with the exact same NOP. This is
somewhat pointless.

Additionally, at module load time we don't call jump_label_apply_nops to
patch the optimal NOPs in, unlike other architectures, but get away with
this because we only use the architectural NOP anyway. A later notifier
will patch NOPs with branches as required.

Similarly to x86 commit 40606294b4fab2ac (x86/jump-label: Do not bother
updating NOPs if they are correct), we can avoid patching NOPs with
identical NOPs. Given that we only use a single NOP encoding, this means
we can NOP-out the body of arch_jump_label_transform_static entirely. As
the default __weak arch_jump_label_transform_static implementation
performs a patch, we must use an empty function to achieve this.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jiang Liu <liuj97@gmail.com>
Cc: Laura Abbott <lauraa@codeaurora.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>