[RFC PATCH 1/2] arm64/tracing: add cntvct based trace clock

[Nicolas Saenz Julienne]

Add a new arm64-specific trace clock using the cntvct register, similar
to x64-tsc. This gives us:
 - A clock that is relatively fast (1GHz on armv8.6, 1-50MHz otherwise),
   monotonic, and resilient to low power modes.
 - It can be used to correlate events across cpus as well as across
   hypervisor and guests.

By using arch_timer_read_counter() we make sure that armv8.6 cpus use
the less expensive CNTVCTSS_EL0, which cannot be accessed speculatively.

Signed-off-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>
---
 arch/arm64/include/asm/trace_clock.h | 12 ++++++++++++
 arch/arm64/kernel/Makefile           |  2 +-
 arch/arm64/kernel/trace_clock.c      | 12 ++++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 arch/arm64/include/asm/trace_clock.h
 create mode 100644 arch/arm64/kernel/trace_clock.c

diff --git a/arch/arm64/include/asm/trace_clock.h b/arch/arm64/include/asm/trace_clock.h
new file mode 100644
index 000000000000..ce4a66d63108
--- /dev/null
+++ b/arch/arm64/include/asm/trace_clock.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_ARM64_TRACE_CLOCK_H
+#define _ASM_ARM64_TRACE_CLOCK_H
+
+#include <linux/types.h>
+
+extern u64 notrace trace_clock_arm64_cntvct(void);
+
+# define ARCH_TRACE_CLOCKS \
+	{ trace_clock_arm64_cntvct, "cntvct", .in_ns = 0 },
+
+#endif  /* _ASM_ARM64_TRACE_CLOCK_H */
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 88b3e2a21408..ec9180f0ac90 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -29,7 +29,7 @@ obj-y			:= debug-monitors.o entry.o irq.o fpsimd.o		\
 			   cpufeature.o alternative.o cacheinfo.o		\
 			   smp.o smp_spin_table.o topology.o smccc-call.o	\
 			   syscall.o proton-pack.o idreg-override.o idle.o	\
-			   patching.o
+			   patching.o trace_clock.o
 
 targets			+= efi-entry.o
 
diff --git a/arch/arm64/kernel/trace_clock.c b/arch/arm64/kernel/trace_clock.c
new file mode 100644
index 000000000000..fe1315f368cb
--- /dev/null
+++ b/arch/arm64/kernel/trace_clock.c
@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * arm64 trace clocks
+ */
+#include <asm/trace_clock.h>
+
+#include <clocksource/arm_arch_timer.h>
+
+u64 notrace trace_clock_arm64_cntvct(void)
+{
+	return arch_timer_read_counter();
+}
-- 
2.33.1