Commits on Dec 1, 2019

1. Fix the implementation of rdtsc in src/mevent_test.c:68 …

   In inline assembly of xhyve/src/mevent_test.c:68, an unused inline
   assembly `__asm__ __volatile__ ("cpuid")` is written here. `cpuid` is a
   instruction which override register eax, ebx, ecx and edx by the
   information of cpu, (details can be found in x86_64 specification).
   This inline assembly is not only unuseful, under some context, this
   inline assembly may corrupt the normal C data flow, and cause errors in
   C world. For example:
   ```
   static __inline uint64_t rdtsc(void)
   {
   	unsigned a, d;
   	__asm__ __volatile__ ("cpuid");
   	__asm__ __volatile__ ("rdtsc" : "=a" (a), "=d" (d));
   
   	return (((uint64_t) a) | (((uint64_t) d) << 32));
   }
   
   int __attribute__((noinline)) set_val(int v) { return v; }
   
   int main() {
     int a = set_val(0);
     int b = set_val(1);
     int c = set_val(2);
     a = a + b; b = b + c; c = c + a;
     uint64_t d = rdtsc();
     printf("%d, %d, %d, %lx\n", a, b, c, d);
     return 0;
   }
   ```
   The above code on my pc (gcc-8 -O2) will output 1, 2, 3, 7bcbf4733e902,
   (assume rdtsc now is 7bcbf4733e902), but the actual output should be 1, 3,
   3, 7bcbf4733e902, the reason is that cpuid override some registers
   unexpectedly.

   

   wierton committed Dec 1, 2019
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