Introduce kern.timecounter.smp_tsc_adjust tunable (disabled by default) and

respective functionality, allowing to synchronize TSC on APs to match BSP's
during boot.  It may be unsafe in general case due to theoretical chance of
later drift if CPUs are using different clock rate or source, but it allows
to use TSC in some cases when difference caused by some initialization bug,
while TSCs are known to increment synchronously.

Reviewed by:	jimharris, kib
MFC after:	1 month

1 files changed, 60 insertions, 3 deletions

diff --git a/sys/x86/x86/tsc.c b/sys/x86/x86/tsc.c
index 0994e9c..2a6c81d 100644
--- a/sys/x86/x86/tsc.c
+++ b/sys/x86/x86/tsc.c
@@ -65,6 +65,11 @@ int	smp_tsc;
 SYSCTL_INT(_kern_timecounter, OID_AUTO, smp_tsc, CTLFLAG_RDTUN, &smp_tsc, 0,
     "Indicates whether the TSC is safe to use in SMP mode");
 TUNABLE_INT("kern.timecounter.smp_tsc", &smp_tsc);
+
+int	smp_tsc_adjust = 0;
+SYSCTL_INT(_kern_timecounter, OID_AUTO, smp_tsc_adjust, CTLFLAG_RDTUN,
+    &smp_tsc_adjust, 0, "Try to adjust TSC on APs to match BSP");
+TUNABLE_INT("kern.timecounter.smp_tsc_adjust", &smp_tsc_adjust);
 #endif
 
 static int	tsc_shift = 1;
@@ -403,25 +408,77 @@ comp_smp_tsc(void *arg)
 		}
 }
 
+static void
+adj_smp_tsc(void *arg)
+{
+	uint64_t *tsc;
+	int64_t d, min, max;
+	u_int cpu = PCPU_GET(cpuid);
+	u_int first, i, size;
+
+	first = CPU_FIRST();
+	if (cpu == first)
+		return;
+	min = INT64_MIN;
+	max = INT64_MAX;
+	size = (mp_maxid + 1) * 3;
+	for (i = 0, tsc = arg; i < N; i++, tsc += size) {
+		d = tsc[first * 3] - tsc[cpu * 3 + 1];
+		if (d > min)
+			min = d;
+		d = tsc[first * 3 + 1] - tsc[cpu * 3 + 2];
+		if (d > min)
+			min = d;
+		d = tsc[first * 3 + 1] - tsc[cpu * 3];
+		if (d < max)
+			max = d;
+		d = tsc[first * 3 + 2] - tsc[cpu * 3 + 1];
+		if (d < max)
+			max = d;
+	}
+	if (min > max)
+		return;
+	d = min / 2 + max / 2;
+	__asm __volatile (
+		"movl $0x10, %%ecx\n\t"
+		"rdmsr\n\t"
+		"addl %%edi, %%eax\n\t"
+		"adcl %%esi, %%edx\n\t"
+		"wrmsr\n"
+		: /* No output */
+		: "D" ((uint32_t)d), "S" ((uint32_t)(d >> 32))
+		: "ax", "cx", "dx", "cc"
+	);
+}
+
 static int
 test_tsc(void)
 {
 	uint64_t *data, *tsc;
-	u_int i, size;
+	u_int i, size, adj;
 
 	if ((!smp_tsc && !tsc_is_invariant) || vm_guest)
 		return (-100);
 	size = (mp_maxid + 1) * 3;
 	data = malloc(sizeof(*data) * size * N, M_TEMP, M_WAITOK);
+	adj = 0;
+retry:
 	for (i = 0, tsc = data; i < N; i++, tsc += size)
 		smp_rendezvous(tsc_read_0, tsc_read_1, tsc_read_2, tsc);
 	smp_tsc = 1;	/* XXX */
 	smp_rendezvous(smp_no_rendevous_barrier, comp_smp_tsc,
 	    smp_no_rendevous_barrier, data);
+	if (!smp_tsc && adj < smp_tsc_adjust) {
+		adj++;
+		smp_rendezvous(smp_no_rendevous_barrier, adj_smp_tsc,
+		    smp_no_rendevous_barrier, data);
+		goto retry;
+	}
 	free(data, M_TEMP);
 	if (bootverbose)
-		printf("SMP: %sed TSC synchronization test\n",
-		    smp_tsc ? "pass" : "fail");
+		printf("SMP: %sed TSC synchronization test%s\n",
+		    smp_tsc ? "pass" : "fail", 
+		    adj > 0 ? " after adjustment" : "");
 	if (smp_tsc && tsc_is_invariant) {
 		switch (cpu_vendor_id) {
 		case CPU_VENDOR_AMD: