LoongArch/ftrace: Add basic support

This patch contains basic ftrace support for LoongArch. Specifically,
function tracer (HAVE_FUNCTION_TRACER), function graph tracer (HAVE_
FUNCTION_GRAPH_TRACER) are implemented following the instructions in
Documentation/trace/ftrace-design.txt.

Use `-pg` makes stub like a child function `void _mcount(void *ra)`.
Thus, it can be seen store RA and alloc stack before `call _mcount`.
Find `alloc stack` at first, and then find `store RA`.

Note that the functions in both inst.c and time.c should not be hooked
with the compiler's -pg option: to prevent infinite self-referencing for
the former, and to ignore early setup stuff for the latter.

Co-developed-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

1 files changed, 73 insertions, 0 deletions

diff --git a/arch/loongarch/kernel/ftrace.c b/arch/loongarch/kernel/ftrace.c
new file mode 100644
index 000000000000..8c3ec1bc7aad
--- /dev/null
+++ b/arch/loongarch/kernel/ftrace.c
@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022 Loongson Technology Corporation Limited
+ */
+
+#include <linux/init.h>
+#include <linux/ftrace.h>
+#include <linux/syscalls.h>
+#include <linux/uaccess.h>
+
+#include <asm/asm.h>
+#include <asm/asm-offsets.h>
+#include <asm/cacheflush.h>
+#include <asm/inst.h>
+#include <asm/loongarch.h>
+#include <asm/syscall.h>
+
+#include <asm-generic/sections.h>
+
+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
+
+/*
+ * As `call _mcount` follows LoongArch psABI, ra-saved operation and
+ * stack operation can be found before this insn.
+ */
+
+static int ftrace_get_parent_ra_addr(unsigned long insn_addr, int *ra_off)
+{
+	int limit = 32;
+	union loongarch_instruction *insn;
+
+	insn = (union loongarch_instruction *)insn_addr;
+
+	do {
+		insn--;
+		limit--;
+
+		if (is_ra_save_ins(insn))
+			*ra_off = -((1 << 12) - insn->reg2i12_format.immediate);
+
+	} while (!is_stack_alloc_ins(insn) && limit);
+
+	if (!limit)
+		return -EINVAL;
+
+	return 0;
+}
+
+void prepare_ftrace_return(unsigned long self_addr,
+		unsigned long callsite_sp, unsigned long old)
+{
+	int ra_off;
+	unsigned long return_hooker = (unsigned long)&return_to_handler;
+
+	if (unlikely(ftrace_graph_is_dead()))
+		return;
+
+	if (unlikely(atomic_read(&current->tracing_graph_pause)))
+		return;
+
+	if (ftrace_get_parent_ra_addr(self_addr, &ra_off))
+		goto out;
+
+	if (!function_graph_enter(old, self_addr, 0, NULL))
+		*(unsigned long *)(callsite_sp + ra_off) = return_hooker;
+
+	return;
+
+out:
+	ftrace_graph_stop();
+	WARN_ON(1);
+}
+#endif	/* CONFIG_FUNCTION_GRAPH_TRACER */