From bec8520dca0d27c1ddac703f9d0a78275ca2603e Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Sat, 19 Nov 2016 13:47:35 +0000 Subject: [PATCH] x86/tsc: Detect random warps If time warps can be observed then they should only ever be observed on one CPU. If they are observed on both CPUs then the system is completely hosed. Add a check for this condition and notify if it happens. Signed-off-by: Thomas Gleixner Reviewed-by: Ingo Molnar Cc: Peter Zijlstra Cc: Yinghai Lu Cc: Borislav Petkov Link: http://lkml.kernel.org/r/20161119134017.574838461@linutronix.de Signed-off-by: Thomas Gleixner --- arch/x86/kernel/tsc_sync.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c index 78083bf23ed1..40f8edd55151 100644 --- a/arch/x86/kernel/tsc_sync.c +++ b/arch/x86/kernel/tsc_sync.c @@ -37,6 +37,7 @@ 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 @@ -45,7 +46,7 @@ static int nr_warps; static void check_tsc_warp(unsigned int timeout) { cycles_t start, now, prev, end; - int i; + int i, cur_warps = 0; start = rdtsc_ordered(); /* @@ -85,7 +86,14 @@ static void check_tsc_warp(unsigned int timeout) if (unlikely(prev > now)) { arch_spin_lock(&sync_lock); max_warp = max(max_warp, prev - now); + /* + * 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); } } @@ -160,6 +168,8 @@ void check_tsc_sync_source(int cpu) 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"); } else { pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n", @@ -170,6 +180,7 @@ void check_tsc_sync_source(int cpu) * 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; -- 2.20.1