First, HVO must have ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU so that
incompatible CPUs can be onlined if HVO is not in use. The
late_cpu_enable() callback is used to check if HVO is truly in use.

If the late CPU is incompatible with HVO and HVO is *not* in use, HVO
becomes permanently disabled.

If the late CPU is incompatible with HVO and HVO *is* in use, the CPU
is not onlined.

If the late CPU is compatible with HVO, there is no issue.

Signed-off-by: James Houghton <jthoughton@google.com>
---
 arch/arm64/kernel/cpufeature.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 7e2c134f3b55..65bb3c7c9595 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -76,6 +76,7 @@
 #include <linux/kasan.h>
 #include <linux/percpu.h>
 #include <linux/sched/isolation.h>
+#include <linux/hugetlb_vmemmap.h>
 
 #include <asm/arm_pmuv3.h>
 #include <asm/cpu.h>
@@ -2181,6 +2182,22 @@ static bool hvo_compatible(const struct arm64_cpu_capabilities *caps, int scope)
 	return supports_hw_af(scope);
 }
 
+static bool late_cpu_enable_hvo(const struct arm64_cpu_capabilities *__unused)
+{
+#ifdef CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
+	if (cpu_has_hw_af())
+		return true;
+
+	/*
+	 * If the CPU does not support HW AF, we cannot online it if HVO is
+	 * currently in use.
+	 */
+	return hugetlb_vmemmap_optimization_try_disable();
+#else
+	return true;
+#endif
+}
+
 static void cpu_enable_pan(const struct arm64_cpu_capabilities *__unused)
 {
 	/*
@@ -3079,8 +3096,11 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 	{
 		.desc = "HugeTLB Vmemmap Optimization Support",
 		.capability = ARM64_HVO_COMPATIBLE,
-		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.type = ARM64_CPUCAP_SCOPE_SYSTEM |
+			ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU |
+			ARM64_CPUCAP_PERMITTED_FOR_LATE_CPU,
 		.matches = hvo_compatible,
+		.late_cpu_enable = late_cpu_enable_hvo,
 	},
 	{
 		.desc = "52-bit Virtual Addressing for KVM (LPA2)",
-- 
2.55.0.795.g602f6c329a-goog