riscv: entry: Save a0 prior syscall_enter_from_user_mode()

The RISC-V calling convention passes the first argument, and the
return value in the a0 register. For this reason, the a0 register
needs some extra care; When handling syscalls, the a0 register is
saved into regs->orig_a0, so a0 can be properly restored for,
e.g. interrupted syscalls.

This functionality was broken with the introduction of the generic
entry patches. Here, a0 was saved into orig_a0 after calling
syscall_enter_from_user_mode(), which can change regs->a0 for some
paths, incorrectly restoring a0.

This is resolved, by saving a0 prior doing the
syscall_enter_from_user_mode() call.

Fixes: f0bddf5 ("riscv: entry: Convert to generic entry")
Reviewed-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reported-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20230403065207.1070974-1-bjorn@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/traps.c

@@ -270,11 +270,11 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
 	if (user_mode(regs)) {
 		ulong syscall = regs->a7;
 
-		syscall = syscall_enter_from_user_mode(regs, syscall);
-
 		regs->epc += 4;
 		regs->orig_a0 = regs->a0;
 
+		syscall = syscall_enter_from_user_mode(regs, syscall);
+
 		if (syscall < NR_syscalls)
 			syscall_handler(regs, syscall);
 		else