MDEV-27429: Support RISC-V cycle timer

Adapted from https://github.com/google/benchmark/pull/833
authored by Sam Elliot at lowRISC.

This requires the RISCV kernel to set the CY bit of the mcountern register
which is done on Linux, but documenting here in case another OS hits
a SIGILL here.

When CY bit of the mcounteren register is unset, reading the cycle register
will cause illegal instruction exception in the next privilege level ( user
mode or supervisor mode ). See the privileged isa manual section 3.1.11 in
https://github.com/riscv/riscv-isa-manual/releases/latest

include/my_rdtsc.h

@@ -91,6 +91,7 @@ C_MODE_START
   On AARCH64, we use the generic timer base register. We override clang
   implementation for aarch64 as it access a PMU register which is not
   guarenteed to be active.
+  On RISC-V, we use the rdcycle instruction to read from mcycle register.
 
   Sadly, we have nothing for the Digital Alpha, MIPS, Motorola m68k,
   HP PA-RISC or other non-mainstream (or obsolete) processors.
@@ -175,6 +176,28 @@ static inline ulonglong my_timer_cycles(void)
     __asm __volatile("mrs	%0, CNTVCT_EL0" : "=&r" (result));
     return result;
   }
+#elif defined(__riscv)
+  /* Use RDCYCLE (and RDCYCLEH on riscv32) */
+  {
+# if __riscv_xlen == 32
+    ulong result_lo, result_hi0, result_hi1;
+    /* Implemented in assembly because Clang insisted on branching. */
+    __asm __volatile__(
+        "rdcycleh %0\n"
+        "rdcycle %1\n"
+        "rdcycleh %2\n"
+        "sub %0, %0, %2\n"
+        "seqz %0, %0\n"
+        "sub %0, zero, %0\n"
+        "and %1, %1, %0\n"
+        : "=r"(result_hi0), "=r"(result_lo), "=r"(result_hi1));
+    return (static_cast<ulonglong>(result_hi1) << 32) | result_lo;
+# else
+    ulonglong result;
+    __asm __volatile__("rdcycle %0" : "=r"(result));
+    return result;
+  }
+# endif
 #elif defined(HAVE_SYS_TIMES_H) && defined(HAVE_GETHRTIME)
   /* gethrtime may appear as either cycle or nanosecond counter */
   return (ulonglong) gethrtime();
@@ -233,6 +256,7 @@ C_MODE_END
 #define MY_TIMER_ROUTINE_GETSYSTEMTIMEASFILETIME 26
 #define MY_TIMER_ROUTINE_ASM_S390                28
 #define MY_TIMER_ROUTINE_AARCH64                 29
+#define MY_TIMER_ROUTINE_RISCV                   30
 
 #endif
 

mysys/my_rdtsc.c

@@ -384,6 +384,8 @@ void my_timer_init(MY_TIMER_INFO *mti)
   mti->cycles.routine= MY_TIMER_ROUTINE_ASM_S390;
 #elif defined(__GNUC__) && defined (__aarch64__)
   mti->cycles.routine= MY_TIMER_ROUTINE_AARCH64;
+#elif defined(__GNUC__) && defined (__riscv)
+  mti->cycles.routine= MY_TIMER_ROUTINE_RISCV;
 #elif defined(HAVE_SYS_TIMES_H) && defined(HAVE_GETHRTIME)
   mti->cycles.routine= MY_TIMER_ROUTINE_GETHRTIME;
 #else