linux-kernel - [RFC v5 2/6] Always Running Timer (ART) correlated clocksource

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Message-Id: <1451911523-8534-3-git-send-email-christopher.s.hall@intel.com>
Date:	Mon,  4 Jan 2016 04:45:19 -0800
From:	"Christopher S. Hall" <christopher.s.hall@...el.com>
To:	tglx@...utronix.de, richardcochran@...il.com, mingo@...hat.com,
	john.stultz@...aro.org, hpa@...or.com, jeffrey.t.kirsher@...el.com
Cc:	"Christopher S. Hall" <christopher.s.hall@...el.com>,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	kevin.b.stanton@...el.com
Subject: [RFC v5 2/6] Always Running Timer (ART) correlated clocksource

On modern Intel systems TSC is derived from the new Always Running Timer
(ART). ART can be captured simultaneous to the capture of
audio and network device clocks, allowing a correlation between timebases
to be constructed. Upon capture, the driver converts the captured ART
value to the appropriate system clock using the correlated clocksource
mechanism.

On systems that support ART a new CPUID leaf (0x15) returns parameters
“m” and “n” such that:

TSC_value = (ART_value * m) / n + k [n >= 2]

[k is an offset that can adjusted by a privileged agent. The
IA32_TSC_ADJUST MSR is an example of an interface to adjust k.
See 17.14.4 of the Intel SDM for more details]

Signed-off-by: Christopher S. Hall <christopher.s.hall@...el.com>
---
 arch/x86/include/asm/cpufeature.h |  2 +-
 arch/x86/include/asm/tsc.h        |  2 ++
 arch/x86/kernel/tsc.c             | 46 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index e4f8010..58c799e 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -85,7 +85,7 @@
 #define X86_FEATURE_P4		( 3*32+ 7) /* "" P4 */
 #define X86_FEATURE_CONSTANT_TSC ( 3*32+ 8) /* TSC ticks at a constant rate */
 #define X86_FEATURE_UP		( 3*32+ 9) /* smp kernel running on up */
-/* free, was #define X86_FEATURE_FXSAVE_LEAK ( 3*32+10) * "" FXSAVE leaks FOP/FIP/FOP */
+#define X86_FEATURE_ART		(3*32+10) /* Platform has always running timer (ART) */
 #define X86_FEATURE_ARCH_PERFMON ( 3*32+11) /* Intel Architectural PerfMon */
 #define X86_FEATURE_PEBS	( 3*32+12) /* Precise-Event Based Sampling */
 #define X86_FEATURE_BTS		( 3*32+13) /* Branch Trace Store */
diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 6d7c547..9474c9c 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -29,6 +29,8 @@ static inline cycles_t get_cycles(void)
 	return rdtsc();
 }
 
+extern struct correlated_cs art_timestamper;
+
 extern void tsc_init(void);
 extern void mark_tsc_unstable(char *reason);
 extern int unsynchronized_tsc(void);
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index c7c4d9c..26dcf63 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -949,10 +949,36 @@ static struct notifier_block time_cpufreq_notifier_block = {
 	.notifier_call  = time_cpufreq_notifier
 };
 
+#define ART_CPUID_LEAF (0x15)
+/* The denominator will never be less that 2 */
+#define ART_MIN_DENOMINATOR (2)
+
+static u32 art_to_tsc_numerator;
+static u32 art_to_tsc_denominator;
+
+/*
+ * If ART is present detect the numerator:denominator to convert to TSC
+ */
+static void detect_art(void)
+{
+	unsigned int unused[2];
+
+	if (boot_cpu_data.cpuid_level >= ART_CPUID_LEAF) {
+		cpuid(ART_CPUID_LEAF, &art_to_tsc_denominator,
+		      &art_to_tsc_numerator, unused, unused+1);
+
+		if (art_to_tsc_denominator >= ART_MIN_DENOMINATOR)
+			set_cpu_cap(&boot_cpu_data, X86_FEATURE_ART);
+	}
+}
+
 static int __init cpufreq_tsc(void)
 {
 	if (!cpu_has_tsc)
 		return 0;
+
+	detect_art();
+
 	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
 		return 0;
 	cpufreq_register_notifier(&time_cpufreq_notifier_block,
@@ -1071,6 +1097,24 @@ int unsynchronized_tsc(void)
 	return 0;
 }
 
+/*
+ * Convert ART to TSC given numerator/denominator found in detect_art()
+ */
+static u64 convert_art_to_tsc(struct correlated_cs *cs, u64 cycles)
+{
+	u64 tmp, res;
+
+	res = (cycles / art_to_tsc_denominator) * art_to_tsc_numerator;
+	tmp = (cycles % art_to_tsc_denominator) * art_to_tsc_numerator;
+	res += tmp / art_to_tsc_denominator;
+
+	return res;
+}
+
+struct correlated_cs art_timestamper = {
+	.convert	= convert_art_to_tsc,
+};
+EXPORT_SYMBOL(art_timestamper);
 
 static void tsc_refine_calibration_work(struct work_struct *work);
 static DECLARE_DELAYED_WORK(tsc_irqwork, tsc_refine_calibration_work);
@@ -1142,6 +1186,8 @@ static void tsc_refine_calibration_work(struct work_struct *work)
 		(unsigned long)tsc_khz % 1000);
 
 out:
+	if (boot_cpu_has(X86_FEATURE_ART))
+		art_timestamper.related_cs = &clocksource_tsc;
 	clocksource_register_khz(&clocksource_tsc, tsc_khz);
 }
 
-- 
2.1.4

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