--- /dev/null
+/*
+ * check TSC synchronization.
+ *
+ * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
+ *
+ * We check whether all boot CPUs have their TSC's synchronized,
+ * print a warning if not and turn off the TSC clock-source.
+ *
+ * The warp-check is point-to-point between two CPUs, the CPU
+ * initiating the bootup is the 'source CPU', the freshly booting
+ * CPU is the 'target CPU'.
+ *
+ * Only two CPUs may participate - they can enter in any order.
+ * ( The serial nature of the boot logic and the CPU hotplug lock
+ * protects against more than 2 CPUs entering this code. )
+ */
+#include <linux/spinlock.h>
+#include <linux/kernel.h>
+#include <linux/smp.h>
+#include <linux/nmi.h>
+#include <asm/tsc.h>
+
+/*
+ * Entry/exit counters that make sure that both CPUs
+ * run the measurement code at once:
+ */
+static atomic_t start_count;
+static atomic_t stop_count;
+
+/*
+ * We use a raw spinlock in this exceptional case, because
+ * we want to have the fastest, inlined, non-debug version
+ * of a critical section, to be able to prove TSC time-warps:
+ */
+static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
+
+static cycles_t last_tsc;
+static cycles_t max_warp;
+static int nr_warps;
+
+/*
+ * TSC-warp measurement loop running on both CPUs:
+ */
+static void check_tsc_warp(unsigned int timeout)
+{
+ cycles_t start, now, prev, end;
+ int i;
+
+ rdtsc_barrier();
+ start = get_cycles();
+ rdtsc_barrier();
+ /*
+ * The measurement runs for 'timeout' msecs:
+ */
+ end = start + (cycles_t) tsc_khz * timeout;
+ now = start;
+
+ for (i = 0; ; i++) {
+ /*
+ * We take the global lock, measure TSC, save the
+ * previous TSC that was measured (possibly on
+ * another CPU) and update the previous TSC timestamp.
+ */
+ arch_spin_lock(&sync_lock);
+ prev = last_tsc;
+ rdtsc_barrier();
+ now = get_cycles();
+ rdtsc_barrier();
+ last_tsc = now;
+ arch_spin_unlock(&sync_lock);
+
+ /*
+ * Be nice every now and then (and also check whether
+ * measurement is done [we also insert a 10 million
+ * loops safety exit, so we dont lock up in case the
+ * TSC readout is totally broken]):
+ */
+ if (unlikely(!(i & 7))) {
+ if (now > end || i > 10000000)
+ break;
+ cpu_relax();
+ touch_nmi_watchdog();
+ }
+ /*
+ * Outside the critical section we can now see whether
+ * we saw a time-warp of the TSC going backwards:
+ */
+ if (unlikely(prev > now)) {
+ arch_spin_lock(&sync_lock);
+ max_warp = max(max_warp, prev - now);
+ nr_warps++;
+ arch_spin_unlock(&sync_lock);
+ }
+ }
+ WARN(!(now-start),
+ "Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
+ now-start, end-start);
+}
+
+/*
+ * If the target CPU coming online doesn't have any of its core-siblings
+ * online, a timeout of 20msec will be used for the TSC-warp measurement
+ * loop. Otherwise a smaller timeout of 2msec will be used, as we have some
+ * information about this socket already (and this information grows as we
+ * have more and more logical-siblings in that socket).
+ *
+ * Ideally we should be able to skip the TSC sync check on the other
+ * core-siblings, if the first logical CPU in a socket passed the sync test.
+ * But as the TSC is per-logical CPU and can potentially be modified wrongly
+ * by the bios, TSC sync test for smaller duration should be able
+ * to catch such errors. Also this will catch the condition where all the
+ * cores in the socket doesn't get reset at the same time.
+ */
+static inline unsigned int loop_timeout(int cpu)
+{
+ return (cpumask_weight(cpu_core_mask(cpu)) > 1) ? 2 : 20;
+}
+
+/*
+ * Source CPU calls into this - it waits for the freshly booted
+ * target CPU to arrive and then starts the measurement:
+ */
+void check_tsc_sync_source(int cpu)
+{
+ int cpus = 2;
+
+ /*
+ * No need to check if we already know that the TSC is not
+ * synchronized:
+ */
+ if (unsynchronized_tsc())
+ return;
+
+ if (tsc_clocksource_reliable) {
+ if (cpu == (nr_cpu_ids-1) || system_state != SYSTEM_BOOTING)
+ pr_info(
+ "Skipped synchronization checks as TSC is reliable.\n");
+ return;
+ }
+
+ /*
+ * Reset it - in case this is a second bootup:
+ */
+ atomic_set(&stop_count, 0);
+
+ /*
+ * Wait for the target to arrive:
+ */
+ while (atomic_read(&start_count) != cpus-1)
+ cpu_relax();
+ /*
+ * Trigger the target to continue into the measurement too:
+ */
+ atomic_inc(&start_count);
+
+ check_tsc_warp(loop_timeout(cpu));
+
+ while (atomic_read(&stop_count) != cpus-1)
+ cpu_relax();
+
+ if (nr_warps) {
+ pr_warning("TSC synchronization [CPU#%d -> CPU#%d]:\n",
+ smp_processor_id(), cpu);
+ pr_warning("Measured %Ld cycles TSC warp between CPUs, "
+ "turning off TSC clock.\n", max_warp);
+ mark_tsc_unstable("check_tsc_sync_source failed");
+ } else {
+ pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
+ smp_processor_id(), cpu);
+ }
+
+ /*
+ * Reset it - just in case we boot another CPU later:
+ */
+ atomic_set(&start_count, 0);
+ nr_warps = 0;
+ max_warp = 0;
+ last_tsc = 0;
+
+ /*
+ * Let the target continue with the bootup:
+ */
+ atomic_inc(&stop_count);
+}
+
+/*
+ * Freshly booted CPUs call into this:
+ */
+void check_tsc_sync_target(void)
+{
+ int cpus = 2;
+
+ if (unsynchronized_tsc() || tsc_clocksource_reliable)
+ return;
+
+ /*
+ * Register this CPU's participation and wait for the
+ * source CPU to start the measurement:
+ */
+ atomic_inc(&start_count);
+ while (atomic_read(&start_count) != cpus)
+ cpu_relax();
+
+ check_tsc_warp(loop_timeout(smp_processor_id()));
+
+ /*
+ * Ok, we are done:
+ */
+ atomic_inc(&stop_count);
+
+ /*
+ * Wait for the source CPU to print stuff:
+ */
+ while (atomic_read(&stop_count) != cpus)
+ cpu_relax();
+}
Code Review / kvmfornfv.git / blobdiff