[PATCH RFC] arm64/ftrace: Define a new arm64 trace clock source based on cntpct_el0 register.

[Amit]

From: Amit Singh Tomar <amittomer25 at gmail.com>

Currenty, Default trace clock("local") for host and guest is out of sync. 
This default clock is based on cntvct_el0 register value and both host/guest
see different values of cntvct_el0.

Intention here is to get a clock whose timestamps are comparable between host/guest.
It can be achieved by using clock based on cntpct_el0 register values(host/guest
see same value of cntpct_el0 register all the time).

Define a new arm64 specific trace clock using the cntpct_el0 register,
similar to x86-tsc. It can be used to correlate trace events across
hosts and guest that can be useful for debuging purpose.

Signed-off-by: Amit Singh Tomar <amittomer25 at gmail.com>
---
 Documentation/trace/ftrace.txt       |  8 +++++++-
 arch/arm64/include/asm/Kbuild        |  1 -
 arch/arm64/include/asm/trace_clock.h | 21 +++++++++++++++++++++
 arch/arm64/kernel/Makefile           |  1 +
 arch/arm64/kernel/trace_clock.c      | 23 +++++++++++++++++++++++
 5 files changed, 52 insertions(+), 2 deletions(-)
 create mode 100644 arch/arm64/include/asm/trace_clock.h
 create mode 100644 arch/arm64/kernel/trace_clock.c

diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
index ef621d3..6420e32 100644
--- a/Documentation/trace/ftrace.txt
+++ b/Documentation/trace/ftrace.txt
@@ -323,7 +323,7 @@ of ftrace. Here is a list of some of the key files:
 	Usual clocks for tracing:
 
 	  # cat trace_clock
-	  [local] global counter x86-tsc
+	  [local] global counter x86-tsc arm64-pct
 
 	  local: Default clock, but may not be in sync across CPUs
 
@@ -351,6 +351,12 @@ of ftrace. Here is a list of some of the key files:
 		  to correlate events across hypervisor/guest if
 		  tb_offset is known.
 
+	arm64-pct: This uses ARM64 Physical Timer Count register
+		   value. This is different from default "local"
+		   clock which usese Virtual Timer Count register.
+		   This is consistent across processors and can be
+		   used to correlate events across host/guest.
+
 	To set a clock, simply echo the clock name into this file.
 
 	  echo global > trace_clock
diff --git a/arch/arm64/include/asm/Kbuild b/arch/arm64/include/asm/Kbuild
index 70fd9ff..8608f9e 100644
--- a/arch/arm64/include/asm/Kbuild
+++ b/arch/arm64/include/asm/Kbuild
@@ -50,7 +50,6 @@ generic-y += switch_to.h
 generic-y += termbits.h
 generic-y += termios.h
 generic-y += topology.h
-generic-y += trace_clock.h
 generic-y += types.h
 generic-y += unaligned.h
 generic-y += user.h
diff --git a/arch/arm64/include/asm/trace_clock.h b/arch/arm64/include/asm/trace_clock.h
new file mode 100644
index 0000000..aef4ccd
--- /dev/null
+++ b/arch/arm64/include/asm/trace_clock.h
@@ -0,0 +1,21 @@
+/*
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License, version 2, as
+ * published by the Free Software Foundation.
+ *
+ * Copyright (C) 2015 Amit Singh Tomar, amittomer25 at gmail.com
+*/
+
+#ifndef _ASM_ARM_TRACE_CLOCK_H
+#define _ASM_ARM_TRACE_CLOCK_H
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+
+extern u64 notrace trace_clock_arm64_pct(void);
+
+# define ARCH_TRACE_CLOCKS \
+	{ trace_clock_arm64_pct,  "arm64-pct",  0},
+
+#endif  /* _ASM_ARM_TRACE_CLOCK_H */
+
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 22dc9bc..d58f885 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -36,6 +36,7 @@ arm64-obj-$(CONFIG_EFI)			+= efi.o efi-stub.o efi-entry.o
 arm64-obj-$(CONFIG_PCI)			+= pci.o
 arm64-obj-$(CONFIG_ARMV8_DEPRECATED)	+= armv8_deprecated.o
 arm64-obj-$(CONFIG_ACPI)		+= acpi.o
+arm64-obj-$(CONFIG_TRACING)             += trace_clock.o
 
 obj-y					+= $(arm64-obj-y) vdso/
 obj-m					+= $(arm64-obj-m)
diff --git a/arch/arm64/kernel/trace_clock.c b/arch/arm64/kernel/trace_clock.c
new file mode 100644
index 0000000..f038977
--- /dev/null
+++ b/arch/arm64/kernel/trace_clock.c
@@ -0,0 +1,23 @@
+/*
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License, version 2, as
+ * published by the Free Software Foundation.
+ *
+ * Copyright (C) 2015 Amit Singh Tomar, amittomer25 at gmail.com
+*/
+
+#include <asm/trace_clock.h>
+
+#define read_cnt_pct(cnt_pct) do {				\
+	__asm__ __volatile("isb; mrs %0, cntpct_el0; isb ;"	\
+	: "=r" (cnt_pct));					\
+} while (0)
+
+u64 notrace trace_clock_arm64_pct(void)
+{
+	u64 cnt_pct;
+
+	read_cnt_pct(cnt_pct);
+	return  cnt_pct;
+}
+
-- 
1.9.1