[PATCH 1/2] riscv: ftrace: Add DYNAMIC_FTRACE_WITH_DIRECT_CALLS support

[guoren at kernel.org]

From: Song Shuai <suagrfillet at gmail.com>

This patch adds DYNAMIC_FTRACE_WITH_DIRECT_CALLS support for RISC-V.

select the DYNAMIC_FTRACE_WITH_DIRECT_CALLS to provide the
register_ftrace_direct[_multi] interfaces allowing users to register
the customed trampoline (direct_caller) as the mcount for one or
more target functions. And modify_ftrace_direct[_multi] are also
provided for modifying direct_caller.

To make the direct_caller and the other ftrace hooks (eg. function/fgraph
tracer, k[ret]probes) co-exist, a temporary register is nominated to
store the address of direct_caller in ftrace_regs_caller. After the
setting of the address direct_caller by direct_ops->func and the
RESTORE_REGS in ftrace_regs_caller, direct_caller will be jumped to
by the `jr` inst.

Signed-off-by: Song Shuai <suagrfillet at gmail.com>
Co-developed-by: Guo Ren <guoren at kernel.org>
Signed-off-by: Guo Ren <guoren at kernel.org>
---
 Remove modification of mcount-dyn.S. (Guo Ren)
---
 arch/riscv/Kconfig              |  1 +
 arch/riscv/include/asm/ftrace.h | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 1d0e5838b11b..2828537abfcd 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -278,6 +278,7 @@ config ARCH_RV64I
 	select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
 	select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && MMU && $(cc-option,-fpatchable-function-entry=8)
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
+	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !PREEMPTION
diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
index 84f856a3286e..d2c159cdfa5c 100644
--- a/arch/riscv/include/asm/ftrace.h
+++ b/arch/riscv/include/asm/ftrace.h
@@ -114,6 +114,32 @@ struct ftrace_regs;
 void ftrace_graph_func(unsigned long ip, unsigned long parent_ip,
 		       struct ftrace_ops *op, struct ftrace_regs *fregs);
 #define ftrace_graph_func ftrace_graph_func
+
+static inline void arch_ftrace_set_direct_caller(struct pt_regs *regs, unsigned long addr)
+{
+		/*
+		 * Backup origin detour target (t0) in t1, and set t0 with addr.
+		 *
+		 * Here is the sample code for riscv direct_caller:
+		 *
+		 * addi sp,sp,-16
+		 * move t0,t1
+		 * ^^^^^^^^^^
+		 * sd   t0,0(sp)
+		 * sd   ra,8(sp)
+		 * call my_direct_func1
+		 * ld   t0,0(sp)
+		 * ld   ra,8(sp)
+		 * addi sp,sp,16
+		 * jr t0
+		 *
+		 * (Set t0 with t1 for continuous detour, because t1 contains origin detour
+		 *  target)
+		 */
+		regs->t1 = regs->t0;
+		regs->t0 = addr;
+}
+
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
 
 #endif /* __ASSEMBLY__ */
-- 
2.36.1