riscv: ftrace: Add DYNAMIC_FTRACE_WITH_DIRECT_CALLS support

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 (e.g.
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.

Add DYNAMIC_FTRACE_WITH_DIRECT_CALLS support for RISC-V.
Signed-off-by: Song Shuai <suagrfillet@gmail.com>
Tested-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Guo Ren <guoren@kernel.org>
Acked-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20231130121531.1178502-4-bjorn@kernel.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -114,6 +114,7 @@ config RISCV
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && MMU && (CLANG_SUPPORTS_DYNAMIC_FTRACE || GCC_SUPPORTS_DYNAMIC_FTRACE)
+	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
 	select HAVE_FUNCTION_GRAPH_TRACER

arch/riscv/include/asm/ftrace.h

@@ -135,6 +135,13 @@ 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)
+{
+		regs->t1 = addr;
+}
+#define arch_ftrace_set_direct_caller(fregs, addr) \
+	__arch_ftrace_set_direct_caller(&(fregs)->regs, addr)
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
 
 #endif /* __ASSEMBLY__ */

arch/riscv/kernel/mcount-dyn.S

@@ -229,6 +229,7 @@ SYM_FUNC_END(ftrace_caller)
 
 #else /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
 SYM_FUNC_START(ftrace_regs_caller)
+	mv	t1, zero
 	SAVE_ABI_REGS 1
 	PREPARE_ARGS
 
@@ -236,7 +237,10 @@ SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)
 	call	ftrace_stub
 
 	RESTORE_ABI_REGS 1
+	bnez	t1, .Ldirect
 	jr	t0
+.Ldirect:
+	jr	t1
 SYM_FUNC_END(ftrace_regs_caller)
 
 SYM_FUNC_START(ftrace_caller)
@@ -250,3 +254,9 @@ SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)
 	jr	t0
 SYM_FUNC_END(ftrace_caller)
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+SYM_CODE_START(ftrace_stub_direct_tramp)
+	jr	t0
+SYM_CODE_END(ftrace_stub_direct_tramp)
+#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */