[patch 3/8] x86/tsc: Store and check TSC ADJUST MSR

[Thomas Gleixner]

The TSC_ADJUST MSR shows whether the TSC has been modified. This is helpful
in a two aspects:

1) It allows to detect BIOS wreckage, where SMM code tries to 'hide' the
   cycles spent by storing the TSC value at SMM entry and restoring it at
   SMM exit. On affected machines the TSCs run slowly out of sync up to the
   point where the clocksource watchdog (if available) detects it.

   The TSC_ADJUST MSR allows to detect the TSC modification before that and
   eventually restore it. This is also important for SoCs which have no
   watchdog clocksource and therefore TSC wreckage cannot be detected and
   acted upon.

2) All threads in a package are required to have the same TSC_ADJUST
   value. Broken BIOSes break that and as a result the TSC synchronization
   check fails.

   The TSC_ADJUST MSR allows to detect the deviation when a CPU comes
   online. If detected set it to the value of an already online CPU in the
   same package. This also allows to reduce the number of sync tests
   because with that in place the test is only required for the first CPU
   in a package.

   In principle all CPUs in a system should have the same TSC_ADJUST value
   even across packages, but with physical CPU hotplug this assumption is
   not true because the TSC starts with power on, so physical hotplug has
   to do some trickery to bring the TSC into sync with already running
   packages, which requires to use an TSC_ADJUST value different from CPUs
   which got powered earlier.

   A final enhancement is the opportunity to compensate for unsynced TSCs
   accross nodes at boot time and make the TSC usable that way. It won't
   help for TSCs which run apart due to frequency skew between packages,
   but this gets detected by the clocksource watchdog later.

The first step toward this is to store the TSC_ADJUST value of a starting
CPU and compare it with the value of an already online CPU in the same
package. If they differ, emit a warning and adjust it to the reference
value. The !SMP version just stores the boot value for later verification.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/tsc.h |    6 +++
 arch/x86/kernel/Makefile   |    2 -
 arch/x86/kernel/tsc.c      |    2 +
 arch/x86/kernel/tsc_sync.c |   88 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 97 insertions(+), 1 deletion(-)

--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -48,6 +48,12 @@ extern int tsc_clocksource_reliable;
 extern void check_tsc_sync_source(int cpu);
 extern void check_tsc_sync_target(void);
 
+#ifdef CONFIG_X86_TSC
+extern void tsc_store_and_check_tsc_adjust(void);
+#else
+static inline void tsc_store_and_check_tsc_adjust(void) { }
+#endif
+
 extern int notsc_setup(char *);
 extern void tsc_save_sched_clock_state(void);
 extern void tsc_restore_sched_clock_state(void);
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -75,7 +75,7 @@ apm-y				:= apm_32.o
 obj-$(CONFIG_APM)		+= apm.o
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_SMP)		+= smpboot.o
-obj-$(CONFIG_SMP)		+= tsc_sync.o
+obj-$(CONFIG_X86_TSC)		+= tsc_sync.o
 obj-$(CONFIG_SMP)		+= setup_percpu.o
 obj-$(CONFIG_X86_MPPARSE)	+= mpparse.o
 obj-y				+= apic/
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1365,6 +1365,8 @@ void __init tsc_init(void)
 
 	if (unsynchronized_tsc())
 		mark_tsc_unstable("TSCs unsynchronized");
+	else
+		tsc_store_and_check_tsc_adjust();
 
 	check_system_tsc_reliable();
 
--- a/arch/x86/kernel/tsc_sync.c
+++ b/arch/x86/kernel/tsc_sync.c
@@ -14,12 +14,95 @@
  * ( The serial nature of the boot logic and the CPU hotplug lock
  *   protects against more than 2 CPUs entering this code. )
  */
+#include <linux/topology.h>
 #include <linux/spinlock.h>
 #include <linux/kernel.h>
 #include <linux/smp.h>
 #include <linux/nmi.h>
 #include <asm/tsc.h>
 
+struct tsc_adjust {
+	s64	bootval;
+	s64	adjusted;
+};
+
+static DEFINE_PER_CPU(struct tsc_adjust, tsc_adjust);
+
+#ifndef CONFIG_SMP
+void __init tsc_store_and_check_tsc_adjust(void)
+{
+	struct tsc_adjust *ref, *cur = this_cpu_ptr(&tsc_adjust);
+	s64 bootval;
+
+	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
+		return;
+
+	rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
+	cur->bootval = bootval;
+	cur->adjusted = bootval;
+	pr_info("TSC ADJUST: Boot CPU%u: %lld\n",cpu,  bootval);
+}
+
+#else /* !CONFIG_SMP */
+
+/*
+ * Store and check the TSC ADJUST MSR if available
+ */
+void tsc_store_and_check_tsc_adjust(void)
+{
+	struct tsc_adjust *ref, *cur = this_cpu_ptr(&tsc_adjust);
+	unsigned int refcpu, cpu = smp_processor_id();
+	s64 bootval;
+
+	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
+		return;
+
+	rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
+	cur->bootval = bootval;
+
+	/*
+	 * Check whether this CPU is the first in a package to come up. In
+	 * this case do not check the boot value against another package
+	 * because the package might have been physically hotplugged, where
+	 * TSC_ADJUST is expected to be different.
+	 */
+	refcpu = cpumask_any_but(topology_core_cpumask(cpu), cpu);
+
+	if (refcpu >= nr_cpu_ids) {
+		/*
+		 * First online CPU in a package stores the boot value in
+		 * the adjustment value. This value might change later via
+		 * the sync mechanism. If that fails we still can yell
+		 * about boot values not being consistent.
+		 */
+		cur->adjusted = bootval;
+		pr_info_once("TSC ADJUST: Boot CPU%u: %lld\n", cpu,  bootval);
+		return;
+	}
+
+	ref = per_cpu_ptr(&tsc_adjust, refcpu);
+	/*
+	 * Compare the boot value and complain if it differs in the
+	 * package.
+	 */
+	if (bootval != ref->bootval) {
+		pr_warn("TSC ADJUST differs: Reference CPU%u: %lld CPU%u: %lld\n",
+			refcpu, ref->bootval, cpu, bootval);
+	}
+	/*
+	 * The TSC_ADJUST values in a package must be the same. If the boot
+	 * value on this newly upcoming CPU differs from the adjustment
+	 * value of the already online CPU in this package, set it to that
+	 * adjusted value.
+	 */
+	if (bootval != ref->adjusted) {
+		pr_warn("TSC ADJUST synchronize: Reference CPU%u: %lld CPU%u: %lld\n",
+			refcpu, ref->adjusted, cpu, bootval);
+		cur->adjusted = ref->adjusted;
+		wrmsrl(MSR_IA32_TSC_ADJUST, ref->adjusted);
+	}
+}
+
 /*
  * Entry/exit counters that make sure that both CPUs
  * run the measurement code at once:
@@ -202,6 +285,9 @@ void check_tsc_sync_target(void)
 	if (unsynchronized_tsc() || tsc_clocksource_reliable)
 		return;
 
+	/* Store and check the TSC ADJUST MSR */
+	tsc_store_and_check_tsc_adjust();
+
 	/*
 	 * Register this CPU's participation and wait for the
 	 * source CPU to start the measurement:
@@ -223,3 +309,5 @@ void check_tsc_sync_target(void)
 	while (atomic_read(&stop_count) != cpus)
 		cpu_relax();
 }
+
+#endif /* CONFIG_SMP */