[SRU][Artful][PATCH 1/3] x86/tsc: Future-proof native_calibrate_tsc()

Joseph Salisbury joseph.salisbury at canonical.com
Thu Feb 1 18:59:27 UTC 2018

From: Len Brown <len.brown at intel.com>

BugLink: http://bugs.launchpad.net/bugs/1744988

If the crystal frequency cannot be determined via CPUID(15).crystal_khz or
the built-in table then native_calibrate_tsc() will still set the
X86_FEATURE_TSC_KNOWN_FREQ flag which prevents the refined TSC calibration.

As a consequence such systems use cpu_khz for the TSC frequency which is
incorrect when cpu_khz != tsc_khz resulting in time drift.

Return early when the crystal frequency cannot be retrieved without setting
the X86_FEATURE_TSC_KNOWN_FREQ flag. This ensures that the refined TSC
calibration is invoked.

[ tglx: Steam-blastered changelog. Sigh ]

Fixes: 4ca4df0b7eb0 ("x86/tsc: Mark TSC frequency determined by CPUID as known")
Signed-off-by: Len Brown <len.brown at intel.com>
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Cc: peterz at infradead.org
Cc: Bin Gao <bin.gao at intel.com>
Cc: stable at vger.kernel.org
Link: https://lkml.kernel.org/r/0fe2503aa7d7fc69137141fc705541a78101d2b9.1513920414.git.len.brown@intel.com

(cherry picked from commit da4ae6c4a0b8dee5a5377a385545d2250fa8cddb)
Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
 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 531cc93..8f5cf85 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -612,6 +612,8 @@ unsigned long native_calibrate_tsc(void)
+	if (crystal_khz == 0)
+		return 0;
 	 * TSC frequency determined by CPUID is a "hardware reported"
 	 * frequency and is the most accurate one so far we have. This

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