[PATCH V6 1/3] riscv: Add perf callchain support

[Mao Han]

This patch add support for perf callchain sampling on riscv platform.
The return address of leaf function is retrieved from pt_regs as
it is not saved in the outmost frame.

Signed-off-by: Mao Han <han_mao@xxxxxxxxx>
Cc: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
Cc: Greentime Hu <green.hu@xxxxxxxxx>
Cc: Palmer Dabbelt <palmer@xxxxxxxxxx>
Cc: linux-riscv <linux-riscv@xxxxxxxxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Guo Ren <guoren@xxxxxxxxxx>
---
 arch/riscv/Makefile                |  3 ++
 arch/riscv/kernel/Makefile         |  3 +-
 arch/riscv/kernel/perf_callchain.c | 95 ++++++++++++++++++++++++++++++++++++++
 arch/riscv/kernel/stacktrace.c     |  2 +-
 4 files changed, 101 insertions(+), 2 deletions(-)
 create mode 100644 arch/riscv/kernel/perf_callchain.c

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 7a117be..946565b 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -54,6 +54,9 @@ endif
 ifeq ($(CONFIG_MODULE_SECTIONS),y)
 	KBUILD_LDFLAGS_MODULE += -T $(srctree)/arch/riscv/kernel/module.lds
 endif
+ifeq ($(CONFIG_PERF_EVENTS),y)
+        KBUILD_CFLAGS += -fno-omit-frame-pointer
+endif
 
 KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-relax)
 
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index 2420d37..b1bea89 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -38,6 +38,7 @@ obj-$(CONFIG_MODULE_SECTIONS)	+= module-sections.o
 obj-$(CONFIG_FUNCTION_TRACER)	+= mcount.o ftrace.o
 obj-$(CONFIG_DYNAMIC_FTRACE)	+= mcount-dyn.o
 
-obj-$(CONFIG_PERF_EVENTS)      += perf_event.o
+obj-$(CONFIG_PERF_EVENTS)	+= perf_event.o
+obj-$(CONFIG_PERF_EVENTS)	+= perf_callchain.o
 
 clean:
diff --git a/arch/riscv/kernel/perf_callchain.c b/arch/riscv/kernel/perf_callchain.c
new file mode 100644
index 0000000..72168e1
--- /dev/null
+++ b/arch/riscv/kernel/perf_callchain.c
@@ -0,0 +1,95 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2019 Hangzhou C-SKY Microsystems co.,ltd. */
+
+#include <linux/perf_event.h>
+#include <linux/uaccess.h>
+
+/* Kernel callchain */
+struct stackframe {
+	unsigned long fp;
+	unsigned long ra;
+};
+
+/*
+ * Get the return address for a single stackframe and return a pointer to the
+ * next frame tail.
+ */
+static unsigned long user_backtrace(struct perf_callchain_entry_ctx *entry,
+			unsigned long fp, unsigned long reg_ra)
+{
+	struct stackframe buftail;
+	unsigned long ra = 0;
+	unsigned long *user_frame_tail =
+			(unsigned long *)(fp - sizeof(struct stackframe));
+
+	/* Check accessibility of one struct frame_tail beyond */
+	if (!access_ok(user_frame_tail, sizeof(buftail)))
+		return 0;
+	if (__copy_from_user_inatomic(&buftail, user_frame_tail,
+				      sizeof(buftail)))
+		return 0;
+
+	if (reg_ra != 0)
+		ra = reg_ra;
+	else
+		ra = buftail.ra;
+
+	fp = buftail.fp;
+	if (ra != 0)
+		perf_callchain_store(entry, ra);
+	else
+		return 0;
+
+	return fp;
+}
+
+/*
+ * This will be called when the target is in user mode
+ * This function will only be called when we use
+ * "PERF_SAMPLE_CALLCHAIN" in
+ * kernel/events/core.c:perf_prepare_sample()
+ *
+ * How to trigger perf_callchain_[user/kernel] :
+ * $ perf record -e cpu-clock --call-graph fp ./program
+ * $ perf report --call-graph
+ *
+ * On RISC-V platform, the program being sampled and the C library
+ * need to be compiled with -fno-omit-frame-pointer, otherwise
+ * the user stack will not contain function frame.
+ */
+void perf_callchain_user(struct perf_callchain_entry_ctx *entry,
+			 struct pt_regs *regs)
+{
+	unsigned long fp = 0;
+
+	/* RISC-V does not support perf in guest mode. */
+	if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
+		return;
+
+	fp = regs->s0;
+	perf_callchain_store(entry, regs->sepc);
+
+	fp = user_backtrace(entry, fp, regs->ra);
+	while (fp && !(fp & 0x3) && entry->nr < entry->max_stack)
+		fp = user_backtrace(entry, fp, 0);
+}
+
+bool fill_callchain(unsigned long pc, void *entry)
+{
+	return perf_callchain_store(entry, pc);
+}
+
+void notrace walk_stackframe(struct task_struct *task,
+	struct pt_regs *regs, bool (*fn)(unsigned long, void *), void *arg);
+void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
+			   struct pt_regs *regs)
+{
+
+	/* RISC-V does not support perf in guest mode. */
+	if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+		pr_warn("RISC-V does not support perf in guest mode!");
+		return;
+	}
+
+	walk_stackframe(NULL, regs, fill_callchain, entry);
+}
diff --git a/arch/riscv/kernel/stacktrace.c b/arch/riscv/kernel/stacktrace.c
index f156427..9b376c1 100644
--- a/arch/riscv/kernel/stacktrace.c
+++ b/arch/riscv/kernel/stacktrace.c
@@ -19,7 +19,7 @@ struct stackframe {
 	unsigned long ra;
 };
 
-static void notrace walk_stackframe(struct task_struct *task,
+void notrace walk_stackframe(struct task_struct *task,
 	struct pt_regs *regs, bool (*fn)(unsigned long, void *), void *arg)
 {
 	unsigned long fp, sp, pc;
-- 
2.7.4