csky: Add perf callchain support

This patch add support for perf callchain sampling on csky platform.
As fp is used to unwind the stack, the program being sampled and the
C library need to be compiled with -mbacktrace for user callchains,
kernel callchains require CONFIG_STACKTRACE = y.

Changelog:
 - Coding convention with Christoph's advice for riscv's.
Signed-off-by: Mao Han <han_mao@c-sky.com>
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Christoph Hellwig <hch@infradead.org>

arch/csky/kernel/Makefile

@@ -9,6 +9,7 @@ obj-$(CONFIG_SMP)			+= smp.o
 obj-$(CONFIG_FUNCTION_TRACER)		+= ftrace.o
 obj-$(CONFIG_STACKTRACE)		+= stacktrace.o
 obj-$(CONFIG_CSKY_PMU_V1)		+= perf_event.o
+obj-$(CONFIG_PERF_EVENTS)		+= perf_callchain.o
 
 ifdef CONFIG_FUNCTION_TRACER
 CFLAGS_REMOVE_ftrace.o = $(CC_FLAGS_FTRACE)

arch/csky/kernel/perf_callchain.c

+// 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 lr;
+};
+
+static int unwind_frame_kernel(struct stackframe *frame)
+{
+	if (kstack_end((void *)frame->fp))
+		return -EPERM;
+	if (frame->fp & 0x3 || frame->fp < TASK_SIZE)
+		return -EPERM;
+
+	*frame = *(struct stackframe *)frame->fp;
+	if (__kernel_text_address(frame->lr)) {
+		int graph = 0;
+
+		frame->lr = ftrace_graph_ret_addr(NULL, &graph, frame->lr,
+				NULL);
+	}
+	return 0;
+}
+
+static void notrace walk_stackframe(struct stackframe *fr,
+			struct perf_callchain_entry_ctx *entry)
+{
+	do {
+		perf_callchain_store(entry, fr->lr);
+	} while (unwind_frame_kernel(fr) >= 0);
+}
+
+/*
+ * 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_lr)
+{
+	struct stackframe buftail;
+	unsigned long lr = 0;
+	unsigned long *user_frame_tail = (unsigned long *)fp;
+
+	/* 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_lr != 0)
+		lr = reg_lr;
+	else
+		lr = buftail.lr;
+
+	fp = buftail.fp;
+	perf_callchain_store(entry, lr);
+
+	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 C-SKY platform, the program being sampled and the C library
+ * need to be compiled with * -mbacktrace, 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;
+
+	/* C-SKY does not support virtualization. */
+	if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
+		return;
+
+	fp = regs->regs[4];
+	perf_callchain_store(entry, regs->pc);
+
+	/*
+	 * While backtrace from leaf function, lr is normally
+	 * not saved inside frame on C-SKY, so get lr from pt_regs
+	 * at the sample point. However, lr value can be incorrect if
+	 * lr is used as temp register
+	 */
+	fp = user_backtrace(entry, fp, regs->lr);
+
+	while (fp && !(fp & 0x3) && entry->nr < entry->max_stack)
+		fp = user_backtrace(entry, fp, 0);
+}
+
+void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
+			   struct pt_regs *regs)
+{
+	struct stackframe fr;
+
+	/* C-SKY does not support virtualization. */
+	if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+		pr_warn("C-SKY does not support perf in guest mode!");
+		return;
+	}
+
+	fr.fp = regs->regs[4];
+	fr.lr = regs->lr;
+	walk_stackframe(&fr, entry);
+}