Implement random number generation with x86 rdrand

kernel/hal/hal.h

@@ -19,6 +19,7 @@ size_t hal_strspn(const char *s, const char *accept);
 char *hal_strcpy(char *dest, const char *src);
 char *hal_strchr(const char *s, int c);
 void hal_wait(uint32_t ms);
+int32_t hal_randnum(void);
 
 #define HAL_PAGE_SIZE 0x1000
 #include "x86_64/vmm.h"

kernel/hal/x86_64/randnum.S

@@ -0,0 +1,13 @@
+.global hal_randnum
+hal_randnum:
+  mov $100, %ecx
+  xor %eax, %eax
+
+.retry:
+  rdrand %eax
+  jc .done
+  loop .retry
+.fail:
+  mov $-1, %eax
+.done:  
+  ret