From: Len Brown len.brown@intel.com

Linux-4.10 added X86_FEATURE_TSC_KNOWN_FREQ.

commit 4ca4df0b7eb0 ("x86/tsc: Mark TSC frequency determined by CPUID as known")

When it is set, run-time refined TSC calibration is disabled.

But native_calibrate_tsc() sets it on all systems with CPUID.15, even if crystal_khz can not be discovered via CPUID.15 or via built-in table. eg. When a new Intel processor comes out that does not return CPUID.15.crystal_khz, and does not have its model# and crystal_khz listed in native_calibate_tsc().

This results in (tsc_khz = cpu_khz) and TSC refined calibration disabled. But for machines with the TSC and CPU in different clock domains (and those support CPUID.15), almost all such configurations will experience measurable negative timeofday clock drift.

Signed-off-by: Len Brown len.brown@intel.com Cc: Bin Gao bin.gao@intel.com Cc: stable@vger.kernel.org # v4.10+ --- arch/x86/kernel/tsc.c | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c index 8ea117f8142e..49d772672367 100644 --- a/arch/x86/kernel/tsc.c +++ b/arch/x86/kernel/tsc.c @@ -609,6 +609,8 @@ unsigned long native_calibrate_tsc(void) case INTEL_FAM6_ATOM_GOLDMONT: crystal_khz = 19200; /* 19.2 MHz */ break; + default: + return 0; /* no X86_FEATURE_TSC_KNOWN_FREQ */ } }