x86/process: Allow runtime control of Speculative Store Bypass

[mirror_ubuntu-artful-kernel.git] / arch / x86 / kernel / tsc_sync.c

/*
 * 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/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;
        unsigned long   nextcheck;
        bool            warned;
};

static DEFINE_PER_CPU(struct tsc_adjust, tsc_adjust);

void tsc_verify_tsc_adjust(bool resume)
{
        struct tsc_adjust *adj = this_cpu_ptr(&tsc_adjust);
        s64 curval;

        if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
                return;

        /* Rate limit the MSR check */
        if (!resume && time_before(jiffies, adj->nextcheck))
                return;

        adj->nextcheck = jiffies + HZ;

        rdmsrl(MSR_IA32_TSC_ADJUST, curval);
        if (adj->adjusted == curval)
                return;

        /* Restore the original value */
        wrmsrl(MSR_IA32_TSC_ADJUST, adj->adjusted);

        if (!adj->warned || resume) {
                pr_warn(FW_BUG "TSC ADJUST differs: CPU%u %lld --> %lld. Restoring\n",
                        smp_processor_id(), adj->adjusted, curval);
                adj->warned = true;
        }
}

static void tsc_sanitize_first_cpu(struct tsc_adjust *cur, s64 bootval,
                                   unsigned int cpu, bool bootcpu)
{
        /*
         * 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.
         *
         * On the boot cpu we just force set the ADJUST value to 0 if it's
         * non zero. We don't do that on non boot cpus because physical
         * hotplug should have set the ADJUST register to a value > 0 so
         * the TSC is in sync with the already running cpus.
         */
        if (bootcpu && bootval != 0) {
                pr_warn(FW_BUG "TSC ADJUST: CPU%u: %lld force to 0\n", cpu,
                        bootval);
                wrmsrl(MSR_IA32_TSC_ADJUST, 0);
                bootval = 0;
        }
        cur->adjusted = bootval;
}

#ifndef CONFIG_SMP
bool __init tsc_store_and_check_tsc_adjust(bool bootcpu)
{
        struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
        s64 bootval;

        if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
                return false;

        rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
        cur->bootval = bootval;
        cur->nextcheck = jiffies + HZ;
        tsc_sanitize_first_cpu(cur, bootval, smp_processor_id(), bootcpu);
        return false;
}

#else /* !CONFIG_SMP */

/*
 * Store and check the TSC ADJUST MSR if available
 */
bool tsc_store_and_check_tsc_adjust(bool bootcpu)
{
        struct tsc_adjust *ref, *cur = this_cpu_ptr(&tsc_adjust);
        unsigned int refcpu, cpu = smp_processor_id();
        struct cpumask *mask;
        s64 bootval;

        if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
                return false;

        rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
        cur->bootval = bootval;
        cur->nextcheck = jiffies + HZ;
        cur->warned = false;

        /*
         * 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 new package might have been physically hotplugged,
         * where TSC_ADJUST is expected to be different. When called on the
         * boot CPU topology_core_cpumask() might not be available yet.
         */
        mask = topology_core_cpumask(cpu);
        refcpu = mask ? cpumask_any_but(mask, cpu) : nr_cpu_ids;

        if (refcpu >= nr_cpu_ids) {
                tsc_sanitize_first_cpu(cur, bootval, smp_processor_id(),
                                       bootcpu);
                return false;
        }

        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(FW_BUG "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);
        }
        /*
         * We have the TSCs forced to be in sync on this package. Skip sync
         * test:
         */
        return true;
}

/*
 * 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;
static atomic_t skip_test;
static atomic_t test_runs;

/*
 * 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;
static int random_warps;

/*
 * TSC-warp measurement loop running on both CPUs.  This is not called
 * if there is no TSC.
 */
static cycles_t check_tsc_warp(unsigned int timeout)
{
        cycles_t start, now, prev, end, cur_max_warp = 0;
        int i, cur_warps = 0;

        start = rdtsc_ordered();
        /*
         * 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;
                now = rdtsc_ordered();
                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);
                        cur_max_warp = max_warp;
                        /*
                         * Check whether this bounces back and forth. Only
                         * one CPU should observe time going backwards.
                         */
                        if (cur_warps != nr_warps)
                                random_warps++;
                        nr_warps++;
                        cur_warps = nr_warps;
                        arch_spin_unlock(&sync_lock);
                }
        }
        WARN(!(now-start),
                "Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
                        now-start, end-start);
        return cur_max_warp;
}

/*
 * 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(topology_core_cpumask(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 or if we have no TSC.
         */
        if (unsynchronized_tsc())
                return;

        /*
         * Set the maximum number of test runs to
         *  1 if the CPU does not provide the TSC_ADJUST MSR
         *  3 if the MSR is available, so the target can try to adjust
         */
        if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
                atomic_set(&test_runs, 1);
        else
                atomic_set(&test_runs, 3);
retry:
        /*
         * Wait for the target to start or to skip the test:
         */
        while (atomic_read(&start_count) != cpus - 1) {
                if (atomic_read(&skip_test) > 0) {
                        atomic_set(&skip_test, 0);
                        return;
                }
                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 the test was successful set the number of runs to zero and
         * stop. If not, decrement the number of runs an check if we can
         * retry. In case of random warps no retry is attempted.
         */
        if (!nr_warps) {
                atomic_set(&test_runs, 0);

                pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
                        smp_processor_id(), cpu);

        } else if (atomic_dec_and_test(&test_runs) || random_warps) {
                /* Force it to 0 if random warps brought us here */
                atomic_set(&test_runs, 0);

                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);
                if (random_warps)
                        pr_warning("TSC warped randomly between CPUs\n");
                mark_tsc_unstable("check_tsc_sync_source failed");
        }

        /*
         * Reset it - just in case we boot another CPU later:
         */
        atomic_set(&start_count, 0);
        random_warps = 0;
        nr_warps = 0;
        max_warp = 0;
        last_tsc = 0;

        /*
         * Let the target continue with the bootup:
         */
        atomic_inc(&stop_count);

        /*
         * Retry, if there is a chance to do so.
         */
        if (atomic_read(&test_runs) > 0)
                goto retry;
}

/*
 * Freshly booted CPUs call into this:
 */
void check_tsc_sync_target(void)
{
        struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
        unsigned int cpu = smp_processor_id();
        cycles_t cur_max_warp, gbl_max_warp;
        int cpus = 2;

        /* Also aborts if there is no TSC. */
        if (unsynchronized_tsc())
                return;

        /*
         * Store, verify and sanitize the TSC adjust register. If
         * successful skip the test.
         *
         * The test is also skipped when the TSC is marked reliable. This
         * is true for SoCs which have no fallback clocksource. On these
         * SoCs the TSC is frequency synchronized, but still the TSC ADJUST
         * register might have been wreckaged by the BIOS..
         */
        if (tsc_store_and_check_tsc_adjust(false) || tsc_clocksource_reliable) {
                atomic_inc(&skip_test);
                return;
        }

retry:
        /*
         * 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();

        cur_max_warp = check_tsc_warp(loop_timeout(cpu));

        /*
         * Store the maximum observed warp value for a potential retry:
         */
        gbl_max_warp = max_warp;

        /*
         * Ok, we are done:
         */
        atomic_inc(&stop_count);

        /*
         * Wait for the source CPU to print stuff:
         */
        while (atomic_read(&stop_count) != cpus)
                cpu_relax();

        /*
         * Reset it for the next sync test:
         */
        atomic_set(&stop_count, 0);

        /*
         * Check the number of remaining test runs. If not zero, the test
         * failed and a retry with adjusted TSC is possible. If zero the
         * test was either successful or failed terminally.
         */
        if (!atomic_read(&test_runs))
                return;

        /*
         * If the warp value of this CPU is 0, then the other CPU
         * observed time going backwards so this TSC was ahead and
         * needs to move backwards.
         */
        if (!cur_max_warp)
                cur_max_warp = -gbl_max_warp;

        /*
         * Add the result to the previous adjustment value.
         *
         * The adjustement value is slightly off by the overhead of the
         * sync mechanism (observed values are ~200 TSC cycles), but this
         * really depends on CPU, node distance and frequency. So
         * compensating for this is hard to get right. Experiments show
         * that the warp is not longer detectable when the observed warp
         * value is used. In the worst case the adjustment needs to go
         * through a 3rd run for fine tuning.
         */
        cur->adjusted += cur_max_warp;

        pr_warn("TSC ADJUST compensate: CPU%u observed %lld warp. Adjust: %lld\n",
                cpu, cur_max_warp, cur->adjusted);

        wrmsrl(MSR_IA32_TSC_ADJUST, cur->adjusted);
        goto retry;

}

#endif /* CONFIG_SMP */