UefiCpuPkg/CpuTimerDxeRiscV64: Add support for Sstc

Sstc extension allows to program the timer and receive the interrupt
without using an SBI call. This reduces the latency to generate the timer
interrupt. So, detect whether Sstc extension is supported and use the
stimecmp register directly to program the timer interrupt.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Andrei Warkentin <andrei.warkentin@intel.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Andrei Warkentin <andrei.warkentin@intel.com>
Reviewed-by: Dhaval Sharma <dhaval@rivosinc.com>

UefiCpuPkg/CpuTimerDxeRiscV64/Timer.c

@@ -44,6 +44,45 @@ STATIC EFI_TIMER_NOTIFY  mTimerNotifyFunction;
 STATIC UINT64  mTimerPeriod     = 0;
 STATIC UINT64  mLastPeriodStart = 0;
 
+//
+// Sstc support
+//
+STATIC BOOLEAN  mSstcEnabled = FALSE;
+
+/**
+  Program the timer.
+
+  Program either using stimecmp (when Sstc extension is enabled) or using SBI
+  TIME call.
+
+  @param NextValue             Core tick value the timer should expire.
+**/
+STATIC
+VOID
+RiscVProgramTimer (
+  UINT64  NextValue
+  )
+{
+  if (mSstcEnabled) {
+    RiscVSetSupervisorTimeCompareRegister (NextValue);
+  } else {
+    SbiSetTimer (NextValue);
+  }
+}
+
+/**
+  Check whether Sstc is enabled in PCD.
+
+**/
+STATIC
+BOOLEAN
+RiscVIsSstcEnabled (
+  VOID
+  )
+{
+  return ((PcdGet64 (PcdRiscVFeatureOverride) & RISCV_CPU_FEATURE_SSTC_BITMASK) != 0);
+}
+
 /**
   Timer Interrupt Handler.
 
@@ -94,7 +133,7 @@ TimerInterruptHandler (
                          ),
                        1000000u
                        );  // convert to tick
-  SbiSetTimer (PeriodStart);
+  RiscVProgramTimer (PeriodStart);
   RiscVEnableTimerInterrupt (); // enable SMode timer int
   gBS->RestoreTPL (OriginalTPL);
 }
@@ -197,8 +236,7 @@ TimerDriverSetTimerPeriod (
                          ),
                        1000000u
                        ); // convert to tick
-  SbiSetTimer (PeriodStart);
-
+  RiscVProgramTimer (PeriodStart);
   mCpu->EnableInterrupt (mCpu);
   RiscVEnableTimerInterrupt (); // enable SMode timer int
   return EFI_SUCCESS;
@@ -282,6 +320,11 @@ TimerDriverInitialize (
   //
   mTimerNotifyFunction = NULL;
 
+  if (RiscVIsSstcEnabled ()) {
+    mSstcEnabled = TRUE;
+    DEBUG ((DEBUG_INFO, "TimerDriverInitialize: Timer interrupt is via Sstc extension\n"));
+  }
+
   //
   // Make sure the Timer Architectural Protocol is not already installed in the system
   //