arm64: Avoid cpus_have_const_cap() for ARM64_HAS_GIC_PRIO_MASKING

arm64: Avoid cpus_have_const_cap() for ARM64_HAS_GIC_PRIO_MASKING

In system_uses_irq_prio_masking() we use cpus_have_const_cap() to check
for ARM64_HAS_GIC_PRIO_MASKING, 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.

When CONFIG_ARM64_PSEUDO_NMI=y the ARM64_HAS_GIC_PRIO_MASKING cpucap is
a strict boot cpu feature which is detected and patched early on the
boot cpu, which both happen in smp_prepare_boot_cpu(). In the window
between the ARM64_HAS_GIC_PRIO_MASKING cpucap is detected and
alternatives are patched we don't run any code that depends upon the
ARM64_HAS_GIC_PRIO_MASKING cpucap:

* We leave DAIF.IF set until after boot alternatives are patched, and
  interrupts are unmasked later in init_IRQ(), so we cannot reach
  IRQ/FIQ entry code and will not use irqs_priority_unmasked().

* We don't call any code which uses arm_cpuidle_save_irq_context() and
  arm_cpuidle_restore_irq_context() during this window.

* We don't call start_thread_common() during this window.

* The local_irq_*() code in <asm/irqflags.h> depends solely on an
  alternative branch since commit:

  a5f61cc636f48bdf ("arm64: irqflags: use alternative branches for pseudo-NMI logic")

  ... and hence will use the default (DAIF-only) masking behaviour until
  alternatives are patched.

* Secondary CPUs are brought up later after alternatives are patched,
  and alternatives are patched on the boot CPU immediately prior to
  calling init_gic_priority_masking(), so we'll correctly initialize
  interrupt masking regardless.

This patch replaces the use of cpus_have_const_cap() with
alternative_has_cap_unlikely(), which avoid generating code to test the
system_cpucaps bitmap and should be better for all subsequent calls at
runtime. As this makes system_uses_irq_prio_masking() equivalent to
__irqflags_uses_pmr(), the latter is removed and replaced with the
former for consistency.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Mark Brown <broonie@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>