tools/power turbostat: Check for non-zero value when MSR probing

For some MSRs, for example, the Platform Energy Counter (RAPL PSYS), it
is required to additionally check for a non-zero value to confirm that
it is present.

From Intel SDM vol. 4:

Platform Energy Counter (R/O)
This MSR is valid only if both platform vendor hardware
implementation and BIOS enablement support it.
This MSR will read 0 if not valid.

Signed-off-by: Patryk Wlazlyn <[e-mail]>
Signed-off-by: Len Brown <[e-mail]>

tools/power/x86/turbostat/turbostat.c

···
 int probe_msr(int cpu, off_t offset)
 {
 	ssize_t retval;
-	unsigned long long dummy;
+	unsigned long long value;
 
 	assert(!no_msr);
 
-	retval = pread(get_msr_fd(cpu), &dummy, sizeof(dummy), offset);
+	retval = pread(get_msr_fd(cpu), &value, sizeof(value), offset);
 
-	if (retval != sizeof(dummy))
+	/*
+	 * Expect MSRs to accumulate some non-zero value since the system was powered on.
+	 * Treat zero as a read failure.
+	 */
+	if (retval != sizeof(value) || value == 0)
 		return 1;
 
 	return 0;