linux-kernel - [PATCH 4.14 40/89] x86/tsc: Future-proof native_calibrate

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Date:   Mon, 22 Jan 2018 09:45:20 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Len Brown <len.brown@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>, peterz@...radead.org,
        Bin Gao <bin.gao@...el.com>
Subject: [PATCH 4.14 40/89] x86/tsc: Future-proof native_calibrate_tsc()

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Len Brown <len.brown@...el.com>

commit da4ae6c4a0b8dee5a5377a385545d2250fa8cddb upstream.

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@...el.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: peterz@...radead.org
Cc: Bin Gao <bin.gao@...el.com>
Link: https://lkml.kernel.org/r/0fe2503aa7d7fc69137141fc705541a78101d2b9.1513920414.git.len.brown@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/kernel/tsc.c |    2 ++
 1 file changed, 2 insertions(+)

--- 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