nx2: run the guest counter at 31.25 MHz for Switch 2 titles

The Switch 2 counter runs at 31.25 MHz and its SDK converts ticks with
that constant: Mario Kart World's sdk builds 31,250,000 in
nn::os::GetSystemTickFrequency, TickManager::ConvertToTimeSpan and
TickManager::ConvertToTick (19.2 MHz appears nowhere). Eglantine counted
at the Switch 1 rate for every title, so every tick-based time in a
Switch 2 game was off by 1.63x: elapsed times read short, and a deadline
of one 60 Hz frame converted to ticks waited ~27 ms.

The wall clock now has a guest counter frequency, set at boot: 31.25 MHz
for Switch 2 programs, 19.2 MHz otherwise, EDEN_NX2_CNTFRQ=<Hz> to force
one. CNTPCT reads, CNTFRQ_EL0, svcGetSystemTick, thread tick counts and
the time services' tick conversions follow it, and the CNTPCT spin
detection keeps its ~1.7 us threshold.

Mario Kart World, title screen and attract demo, 180 s, same build:
frame times unchanged (17-19 ms once warm), process CPU time 848 s
instead of 930 s.

1 files changed, 26 insertions, 5 deletions

diff --git a/src/common/cpu_features.cpp b/src/common/cpu_features.cpp
index 52fd003c44..93eecb1dc5 100644
--- a/src/common/cpu_features.cpp
+++ b/src/common/cpu_features.cpp
@@ -278,8 +278,16 @@ std::chrono::milliseconds WallClock::GetTimeMS() const {
 
 s64 WallClock::GetCNTPCT() const {
     if (!invariant)
-        return GetUptime() * NsToCNTPCTRatio::num / NsToCNTPCTRatio::den;
-    return MultiplyHigh(GetUptime(), cntpct_rdtsc_factor);
+        return MultiplyAndDivide64(GetUptime(), GetGuestCNTFRQ(), NsRatio::den);
+    return MultiplyHigh(GetUptime(), cntpct_rdtsc_factor.load(std::memory_order_relaxed));
+}
+
+void WallClock::SetGuestCNTFRQ(u64 frequency) {
+    guest_cntfrq.store(frequency, std::memory_order_relaxed);
+    if (invariant) {
+        cntpct_rdtsc_factor.store(GetFixedPoint64Factor(frequency, rdtsc_frequency),
+                                  std::memory_order_relaxed);
+    }
 }
 
 s64 WallClock::GetGPUTick() const {
@@ -336,7 +344,7 @@ namespace {
 } // namespace
 
 WallClock::WallClock(bool invariant_, u64 rdtsc_frequency_) noexcept {
-    const u64 host_cntfrq = std::max<u64>(GetHostCNTFRQ(), 1);
+    host_cntfrq = std::max<u64>(GetHostCNTFRQ(), 1);
     ns_cntfrq_factor = GetFixedPointFactor(NsRatio::den, host_cntfrq);
     us_cntfrq_factor = GetFixedPointFactor(UsRatio::den, host_cntfrq);
     ms_cntfrq_factor = GetFixedPointFactor(MsRatio::den, host_cntfrq);
@@ -361,6 +369,11 @@ s64 WallClock::GetCNTPCT() const {
     return MultiplyHigh(GetUptime(), guest_cntfrq_factor);
 }
 
+void WallClock::SetGuestCNTFRQ(u64 frequency) {
+    guest_cntfrq.store(frequency, std::memory_order_relaxed);
+    guest_cntfrq_factor = GetFixedPointFactor(frequency, host_cntfrq);
+}
+
 s64 WallClock::GetGPUTick() const {
     return MultiplyHigh(GetUptime(), gputick_cntfrq_factor);
 }
@@ -399,7 +412,11 @@ std::chrono::milliseconds WallClock::GetTimeMS() const {
 }
 
 s64 WallClock::GetCNTPCT() const {
-    return GetUptime() * NsToCNTPCTRatio::num / NsToCNTPCTRatio::den;
+    return MultiplyAndDivide64(GetUptime(), GetGuestCNTFRQ(), NsRatio::den);
+}
+
+void WallClock::SetGuestCNTFRQ(u64 frequency) {
+    guest_cntfrq.store(frequency, std::memory_order_relaxed);
 }
 
 s64 WallClock::GetGPUTick() const {
@@ -419,9 +436,13 @@ u64 WallClock::NsToTicks(std::chrono::nanoseconds ns) const {
 }
 #endif
 
+u64 WallClock::CPUTickToGuestCNTPCT(u64 cpu_tick) const {
+    return MultiplyAndDivide64(cpu_tick, GetGuestCNTFRQ(), CPUTickFreq);
+}
+
 // Wall clock MUST be initialized AFTER g_cpu_caps
 // C++ only guarantees ctor init in the order they appear in TU
-const WallClock g_wall_clock = [] {
+WallClock g_wall_clock = [] {
 #if defined(ARCHITECTURE_x86_64)
     auto const& caps = Common::g_cpu_caps;
     return WallClock(caps.invariant_tsc && caps.tsc_frequency > std::nano::den, caps.tsc_frequency);