arm64: Avoid cpus_have_const_cap() for ARM64_WORKAROUND_NVIDIA_CARMEL_CNP

In has_useable_cnp() we use cpus_have_const_cap() to check for
ARM64_WORKAROUND_NVIDIA_CARMEL_CNP, but this is not necessary and
cpus_have_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.

We use has_useable_cnp() to determine whether we have the system-wide
ARM64_HAS_CNP cpucap. Due to the structure of the cpufeature code, we
call has_useable_cnp() in two distinct cases:

1) When finalizing system capabilities, setup_system_capabilities() will
   call has_useable_cnp() with SCOPE_SYSTEM to determine whether all
   CPUs have the feature. This is called after we've detected any local
   cpucaps including ARM64_WORKAROUND_NVIDIA_CARMEL_CNP, but prior to
   patching alternatives.

   If the ARM64_WORKAROUND_NVIDIA_CARMEL_CNP was detected, we will not
   detect ARM64_HAS_CNP.

2) After finalizing system capabilties, verify_local_cpu_capabilities()
   will call has_useable_cnp() with SCOPE_LOCAL_CPU to verify that CPUs
   have CNP if we previously detected it.

   Note that if ARM64_WORKAROUND_NVIDIA_CARMEL_CNP was detected, we will
   not have detected ARM64_HAS_CNP.

For case 1 we must check the system_cpucaps bitmap as this occurs prior
to patching the alternatives. For case 2 we'll only call
has_useable_cnp() once per subsequent onlining of a CPU, and as this
isn't a fast path it's not necessary to optimize for this case.

This patch replaces the use of cpus_have_const_cap() with
cpus_have_cap(), which will only generate the bitmap test and avoid
generating an alternative sequence, resulting in slightly simpler annd
smaller code being generated. The ARM64_WORKAROUND_NVIDIA_CARMEL_CNP
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: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: mrutland-arm

arch/arm64/include/asm/cpucaps.h

@@ -54,6 +54,8 @@ cpucap_is_possible(const unsigned int cap)
 		return IS_ENABLED(CONFIG_ARM64_ERRATUM_2658417);
 	case ARM64_WORKAROUND_CAVIUM_23154:
 		return IS_ENABLED(CONFIG_CAVIUM_ERRATUM_23154);
+	case ARM64_WORKAROUND_NVIDIA_CARMEL_CNP:
+		return IS_ENABLED(CONFIG_NVIDIA_CARMEL_CNP_ERRATUM);
 	}
 
 	return true;

arch/arm64/kernel/cpufeature.c

@@ -1629,7 +1629,7 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
 	if (is_kdump_kernel())
 		return false;
 
-	if (cpus_have_const_cap(ARM64_WORKAROUND_NVIDIA_CARMEL_CNP))
+	if (cpus_have_cap(ARM64_WORKAROUND_NVIDIA_CARMEL_CNP))
 		return false;
 
 	return has_cpuid_feature(entry, scope);