[PATCH 4/5] x86/kernel: Provide a means to disable TSC ART

[<mike.travis@hpe.com>]

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On systems where multiple chassis are reset asynchronously, and thus
the TSC counters are started asynchronously, the offset needed to
convert to TSC to ART would be different.  Disable ART in that case
and rely on the TSC counters to supply the accurate time.

Signed-off-by: Mike Travis <mike.travis@hpe.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
---
v2: changed to use common "tsc_async_resets" flag.
---
 arch/x86/kernel/tsc.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- linux.orig/arch/x86/kernel/tsc.c
+++ linux/arch/x86/kernel/tsc.c
@@ -966,10 +966,14 @@ static void detect_art(void)
 	if (boot_cpu_data.cpuid_level < ART_CPUID_LEAF)
 		return;
 
-	/* Don't enable ART in a VM, non-stop TSC and TSC_ADJUST required */
+	/*
+	 * Don't enable ART in a VM, non-stop TSC and TSC_ADJUST required,
+	 * and the TSC counter resets must not occur asynchronously.
+	 */
 	if (boot_cpu_has(X86_FEATURE_HYPERVISOR) ||
 	    !boot_cpu_has(X86_FEATURE_NONSTOP_TSC) ||
-	    !boot_cpu_has(X86_FEATURE_TSC_ADJUST))
+	    !boot_cpu_has(X86_FEATURE_TSC_ADJUST) ||
+	    tsc_async_resets)
 		return;
 
 	cpuid(ART_CPUID_LEAF, &art_to_tsc_denominator,

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