winapi: Calculate TSC frequency on startup

lib/winapi/profiling.c

@@ -3,10 +3,14 @@
 // SPDX-FileCopyrightText: 2019-2025 Stefan Schmidt
 
 #include <assert.h>
+#include <intrin.h>
 #include <profileapi.h>
 #include <stddef.h>
+#include <stdint.h>
 #include <xboxkrnl/xboxkrnl.h>
 
+static LARGE_INTEGER tsc_freq = {.QuadPart = 733333333};
+
 BOOL QueryPerformanceCounter (LARGE_INTEGER *lpPerformanceCount)
 {
     assert(lpPerformanceCount != NULL);
@@ -19,6 +23,75 @@ BOOL QueryPerformanceFrequency (LARGE_INTEGER *lpFrequency)
 {
     assert(lpFrequency != NULL);
 
-    lpFrequency->QuadPart = 733333333;
+    *lpFrequency = tsc_freq;
     return TRUE;
 }
+
+static uint64_t get_cpu_multiplier (void)
+{
+    // Read EBL_CR_POWERON
+    const uint64_t poweron_reg = __readmsr(0x2A);
+
+    // Chapter 2.22 of https://www.intel.com/content/dam/develop/external/us/en/documents/335592-sdm-vol-4.pdf
+    const uint64_t multiplier_pattern = (poweron_reg >> 22) & 0b101111;
+
+    // https://web.archive.org/web/20040407043044/http://www.intel.com:80/design/mobile/datashts/29834006.pdf
+    // clang-format off
+    switch (multiplier_pattern) {
+        case 0b000001: return 30;
+        case 0b000101: return 35;
+        case 0b000010: return 40;
+        case 0b000110: return 45;
+        case 0b000000: return 50;
+        case 0b000100: return 55;
+        case 0b001011: return 60;
+        case 0b001111: return 65;
+        case 0b001001: return 70;
+        case 0b001101: return 75;
+        case 0b001010: return 80;
+        case 0b100110: return 85;
+        case 0b100000: return 90;
+        case 0b100100: return 95;
+        case 0b101011: return 100;
+        case 0b101111: return 105;
+        case 0b101010: return 130;
+        case 0b101100: return 140;
+        // This table is incomplete, the following CPUs and their multipliers are not covered:
+        // 110: Coppermine Celeron 733 SL4P3 SL5E9 SL4P7 SL52Y
+        // 115: Coppermine Celeron 766 SL4P6 SL4QF SL52X SL5EA
+        // 120: Tualatin Celeron 1200 SL5XS SL5Y5 SL656 SL68P SL6C8 SL6JS SL6RP
+        // 150: Tualatin Celeron 1500 SL6C5
+        default: return 0;
+    }
+    // clang-format on
+}
+
+// Gets run by the CRT on startup
+static __cdecl VOID tsc_freq_init (VOID)
+{
+    // NB clock input frequency, in 0.001 Hz units for precision
+    const uint64_t xtal_freq_mhz = 16666666666;
+
+    // CPU-internal clock multiplier, in 0.1 units due to non-integer values
+    const uint64_t multiplier_10x = get_cpu_multiplier();
+
+    if (multiplier_10x == 0) {
+        // Could not determine clock multiplier, fall back to default TSC freq
+        return;
+    }
+
+    uint32_t fsbreg;
+    HalReadWritePCISpace(0, 0x60, 0x6C, &fsbreg, sizeof(fsbreg), FALSE);
+    const uint64_t fsb_divider = fsbreg & 0xFF;
+    const uint64_t fsb_multiplier = (fsbreg >> 8) & 0xFF;
+    if (fsb_divider == 0 || fsb_multiplier == 0) {
+        return;
+    }
+    const uint64_t fsb_freq = (xtal_freq_mhz / fsb_divider) * fsb_multiplier;
+    const uint64_t cpu_freq = fsb_freq * multiplier_10x / 10 / 1000;
+
+    tsc_freq.QuadPart = cpu_freq;
+}
+
+#pragma comment(linker, "/include:___tsc_freq_init_p")
+__attribute__((section(".CRT$XXT"), used)) void(__cdecl *const __tsc_freq_init_p)(void) = tsc_freq_init;