[a64] Implement raw clock with CNTVCT_EL0

Implements a raw clock by directly accessing the `CNTVCT_EL0` and
`CNTFRQ_EL0` registers by using `mrs` intrinsics.

Should allow for a raw-clock to work on all ARM platforms without having
to make a per-platform implementation for any further a64 platforms like
MacOS or Linux or Android and such.

src/xenia/base/clock.cc

@@ -26,8 +26,9 @@ DEFINE_bool(clock_no_scaling, false,
             "Guest system time is directly pulled from host.",
             "CPU");
 DEFINE_bool(clock_source_raw, false,
-            "Use the RDTSC instruction as the time source. "
-            "Host CPU must support invariant TSC.",
+            "On x64, Use the RDTSC instruction as the time source. Requires "
+            "invariant TSC. "
+            "On a64, Use the CNTVCT_EL0 register as the time source",
             "CPU");
 
 namespace xe {

src/xenia/base/clock.h

@@ -16,7 +16,7 @@
 #include "xenia/base/cvar.h"
 #include "xenia/base/platform.h"
 
-#if XE_ARCH_AMD64
+#if XE_ARCH_AMD64 || XE_ARCH_ARM64
 #define XE_CLOCK_RAW_AVAILABLE 1
 #endif
 

src/xenia/base/clock_arm64.cc

@@ -0,0 +1,37 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2026 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#include "xenia/base/clock.h"
+#include "xenia/base/platform.h"
+
+#if XE_ARCH_ARM64 && XE_CLOCK_RAW_AVAILABLE
+
+// Wrap all these different cpu compiler intrinsics.
+// So no inline assembler here and the compiler will remove the clutter.
+#if XE_COMPILER_MSVC
+#include <intrin.h>
+constexpr uint32_t CNTFRQ_EL0 = ARM64_SYSREG(3, 3, 14, 0, 0);
+constexpr uint32_t CNTVCT_EL0 = ARM64_SYSREG(3, 3, 14, 0, 2);
+#define xe_cpu_mrs(reg) _ReadStatusReg(reg)
+#elif XE_COMPILER_CLANG || XE_COMPILER_GNUC
+#include <arm_acle.h>
+#define xe_cpu_mrs(reg) __arm_rsr64(#reg)
+#else
+#error "No cpu instruction wrappers for current compiler implemented."
+#endif
+
+namespace xe {
+XE_NOINLINE
+uint64_t Clock::host_tick_frequency_raw() { return xe_cpu_mrs(CNTFRQ_EL0); }
+XE_NOINLINE
+uint64_t Clock::host_tick_count_raw() { return xe_cpu_mrs(CNTVCT_EL0); }
+
+}  // namespace xe
+
+#endif