Kernel: Detect Aarch64 physical address bit width with CPU ID registers

Author: https://github.com/konradekk
Commit: https://github.com/SerenityOS/serenity/commit/401fc6afae4
Pull-request: https://github.com/SerenityOS/serenity/pull/16781
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/gmta

Kernel/Arch/aarch64/CPUID.cpp

@@ -1478,4 +1478,30 @@ NonnullOwnPtr<KString> build_cpu_feature_names(CPUFeature::Type const& features)
     return KString::must_create(builder.string_view());
 }
 
+u8 detect_physical_address_bit_width()
+{
+    auto memory_model_feature_register_0 = Aarch64::ID_AA64MMFR0_EL1::read();
+
+    switch (memory_model_feature_register_0.PARange) {
+    case 0b0000:
+        return 32; // 4GB
+    case 0b0001:
+        return 36; // 64GB
+    case 0b0010:
+        return 40; // 1TB
+    case 0b0011:
+        return 42; // 4TB
+    case 0b0100:
+        return 44; // 16TB
+    case 0b0101:
+        return 48; // 256TB
+    case 0b0110:
+        return 52; // 4PB (applies for FEAT_LPA or FEAT_LPA2)
+    case 0b0111:
+        return 56; // 64PB (applies for FEAT_D128)
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
 }

Kernel/Arch/aarch64/CPUID.h

@@ -277,4 +277,6 @@ StringView cpu_feature_to_name(CPUFeature::Type const&);
 StringView cpu_feature_to_description(CPUFeature::Type const&);
 NonnullOwnPtr<KString> build_cpu_feature_names(CPUFeature::Type const&);
 
+u8 detect_physical_address_bit_width();
+
 }

Kernel/Arch/aarch64/Processor.cpp

@@ -33,6 +33,7 @@ void Processor::install(u32 cpu)
     VERIFY(g_current_processor == nullptr);
     m_cpu = cpu;
     m_features = detect_cpu_features();
+    m_physical_address_bit_width = detect_physical_address_bit_width();
 
     initialize_exceptions(cpu);
 
@@ -42,6 +43,7 @@ void Processor::install(u32 cpu)
 void Processor::initialize()
 {
     dmesgln("CPU[{}]: Supports {}", m_cpu, build_cpu_feature_names(m_features));
+    dmesgln("CPU[{}]: Physical address bit width: {}", m_cpu, m_physical_address_bit_width);
 }
 
 [[noreturn]] void Processor::halt()

Kernel/Arch/aarch64/Processor.h

@@ -84,8 +84,7 @@ public:
 
     ALWAYS_INLINE u8 physical_address_bit_width() const
     {
-        TODO_AARCH64();
-        return 0;
+        return m_physical_address_bit_width;
     }
 
     ALWAYS_INLINE u8 virtual_address_bit_width() const
@@ -286,6 +285,7 @@ private:
 
     u32 m_cpu;
     CPUFeature::Type m_features;
+    u8 m_physical_address_bit_width;
 
     Thread* m_current_thread;
     Thread* m_idle_thread;