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

Author: https://github.com/konradekk
Commit: https://github.com/SerenityOS/serenity/commit/5791072280a
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

@@ -1504,4 +1504,20 @@ u8 detect_physical_address_bit_width()
     }
 }
 
+u8 detect_virtual_address_bit_width()
+{
+    auto memory_model_feature_register_2 = Aarch64::ID_AA64MMFR2_EL1::read();
+
+    switch (memory_model_feature_register_2.VARange) {
+    case 0b0000:
+        return 48; // 256TB
+    case 0b0001:
+        return 52; // 4PB (only for 64KB translation granule)
+    case 0b0010:
+        return 56; // 64PB (applies for FEAT_D128)
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
 }

Kernel/Arch/aarch64/CPUID.h

@@ -278,5 +278,6 @@ StringView cpu_feature_to_description(CPUFeature::Type const&);
 NonnullOwnPtr<KString> build_cpu_feature_names(CPUFeature::Type const&);
 
 u8 detect_physical_address_bit_width();
+u8 detect_virtual_address_bit_width();
 
 }

Kernel/Arch/aarch64/Processor.cpp

@@ -34,6 +34,7 @@ void Processor::install(u32 cpu)
     m_cpu = cpu;
     m_features = detect_cpu_features();
     m_physical_address_bit_width = detect_physical_address_bit_width();
+    m_virtual_address_bit_width = detect_virtual_address_bit_width();
 
     initialize_exceptions(cpu);
 
@@ -44,6 +45,7 @@ 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);
+    dmesgln("CPU[{}]: Virtual address bit width: {}", m_cpu, m_virtual_address_bit_width);
 }
 
 [[noreturn]] void Processor::halt()

Kernel/Arch/aarch64/Processor.h

@@ -89,8 +89,7 @@ public:
 
     ALWAYS_INLINE u8 virtual_address_bit_width() const
     {
-        TODO_AARCH64();
-        return 0;
+        return m_virtual_address_bit_width;
     }
 
     ALWAYS_INLINE static bool is_initialized()
@@ -286,6 +285,7 @@ private:
     u32 m_cpu;
     CPUFeature::Type m_features;
     u8 m_physical_address_bit_width;
+    u8 m_virtual_address_bit_width;
 
     Thread* m_current_thread;
     Thread* m_idle_thread;