Check for and use RDRAND instruction on Intel/AMD CPUs

randcheck.c

// Compile and run via:
// g++ -o randcheck randcheck.c && ./randcheck

#include <stdlib.h>
#include <stdio.h>

typedef unsigned long long quadword;

quadword check_cpu_features(int leaf_ID) {
  quadword cpu_features = 0;
  asm("xorq %%rcx, %%rcx;"
      "xorq %%rdx, %%rdx;"
      "mov %1, %%eax;"
      "cpuid;"
      // Combine EDX:ECX together, with EDX making up upper 32-bits
      "shlq $32, %%rdx;"
      "orq %%rcx, %%rdx;"
      "movq %%rdx, %0;"
      : "=r" (cpu_features)
      : "r" (leaf_ID)
      : "%rax", "%rcx", "%rdx"
      );
  return cpu_features;
}

void sample_rng(int amount) {
  printf("Actually trying RDRAND instruction now...\n");
  for (int total = 0; total < amount; total += 8) {
    quadword value;
    asm("rdrand %%rax;"
	"movq %%rax, %0;"
	: "=r" (value)
	:
	: "%rax"
	);
    printf("%016llx\n", value);
  }
}

int check_for_bit(quadword value, int index) {
  return ((1LL << index) & value) != 0;
}

int main(int argc, char **argv) {
  quadword features = check_cpu_features(1);
  printf("Advertised CPU features: %016llx\n", features);

  int on_hypervisor = check_for_bit(features,31);
  printf("Running on hypervisor: %s\n", on_hypervisor ? "YES" : "NO");

  int has_rdrand = check_for_bit(features, 30);
  
  if (has_rdrand) {
    printf("CPUID advertises support for RNG.\n");
    sample_rng(64);
  } else {
    printf("This processor does not have a hardware RNG.\n");
  }

  return !has_rdrand;
}