CE implement stall command, libsystem Add stall_ms ()

libsystem/amd64/src.mk

@@ -1,6 +1,8 @@
-c += amd64/syscall.c
+c += amd64/syscall.c \
+		 amd64/stall.c
 
 S += amd64/_start.S
 
 o += amd64/_start.o \
-		 amd64/syscall.o
+		 amd64/syscall.o \
+		 amd64/stall.o

libsystem/amd64/stall.c

@@ -0,0 +1,30 @@
+#include <stall.h>
+#include <stdint.h>
+
+static uint64_t stall_read_tsc (void) {
+  uint32_t lo, hi;
+  __asm__ volatile ("rdtsc" : "=a"(lo), "=d"(hi));
+  return ((uint64_t)hi << 32) | lo;
+}
+
+static uint64_t stall_get_tsc_freq_hz (void) {
+  uint32_t eax, ebx, ecx, edx;
+  __asm__ volatile ("cpuid" : "=a"(eax), "=b"(ebx), "=c"(ecx), "=d"(edx) : "a"(0x15));
+
+  if (eax == 0 || ebx == 0 || ecx == 0)
+    return 2500000000ULL;
+
+  return (uint64_t)ecx * ebx / eax;
+}
+
+void stall_ms (uint64_t ms) {
+  uint64_t freq_hz = stall_get_tsc_freq_hz ();
+
+  uint64_t cycles = freq_hz / 1000;
+  uint64_t wait_cycles = ms * cycles;
+
+  uint64_t now = stall_read_tsc ();
+
+  while ((stall_read_tsc () - now) < wait_cycles)
+    __asm__ volatile ("pause" ::: "memory");
+}

libsystem/stall.h

@@ -0,0 +1,8 @@
+#ifndef _LIBSYSTEM_STALL_H
+#define _LIBSYSTEM_STALL_H
+
+#include <stdint.h>
+
+void stall_ms (uint64_t ms);
+
+#endif // _LIBSYSTEM_STALL_H