In gic_read_iar() we use cpus_have_const_cap() to check for
ARM64_WORKAROUND_CAVIUM_23154 but this is not necessary and
alternative_has_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_WORKAROUND_CAVIUM_23154 cpucap is detected and patched early
on the boot CPU before the GICv3 driver is initialized and hence before
gic_read_iar() is ever called. Thus it is not necessary to use
cpus_have_const_cap(), and alternative_has_cap() is equivalent.

In addition, arm64's gic_read_iar() lives in irq-gic-v3.c purely for
historical reasons. It was originally added prior to 32-bit arm support
in commit:

  6d4e11c5 ("irqchip/gicv3: Workaround for Cavium ThunderX erratum 23154")

When support for 32-bit arm was added, 32-bit arm's gic_read_iar()
implementation was placed in <asm/arch_gicv3.h>, but the arm64 version
was kept within irq-gic-v3.c as it depended on a static key local to
irq-gic-v3.c and it was easier to add ifdeffery, which is what we did in
commit:

  7936e914 ("irqchip/gic-v3: Refactor the arm64 specific parts")

Subsequently the static key was replaced with a cpucap in commit:

  a4023f68 ("arm64: Add hypervisor safe helper for checking constant capabilities")

Since that commit there has been no need to keep arm64's gic_read_iar()
in irq-gic-v3.c.

This patch replaces the use of cpus_have_const_cap() with
alternative_has_cap_unlikely(), which will avoid generating code to test
the system_cpucaps bitmap and should be better for all subsequent calls
at runtime. For consistency, move the arm64-specific gic_read_iar()
implementation over to arm64's <asm/arch_gicv3.h>. The
ARM64_WORKAROUND_CAVIUM_23154 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: Marc Zyngier <maz@kernel.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>