[PATCH v21 11/15] arm64: syscall: Simplify el0_svc_common() syscall exit path

[Jinjie Ruan]

The fast exit path of el0_svc_common() is only taken when no syscall
work was pending on entry and CONFIG_DEBUG_RSEQ is disabled:

	if (!(flags & _TIF_SYSCALL_WORK) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
		flags = read_thread_flags();
		if (unlikely(flags & _TIF_SYSCALL_EXIT_WORK) || flags & _TIF_SINGLESTEP)
			arm64_syscall_exit_to_user_mode_work(regs);
		return;
	}
	trace_exit:
		arm64_syscall_exit_to_user_mode_work(regs);

arm64_syscall_exit_to_user_mode_work() already re-reads the thread
flags and checks the same flags itself. The only other thing it does is
rseq_syscall(), which is an empty stub when CONFIG_DEBUG_RSEQ is
disabled, i.e. whenever the fast path is taken. So both paths end up
doing exactly the same thing, and the fast path can be dropped:

	trace_exit:
		arm64_syscall_exit_to_user_mode_work(regs);

With CONFIG_DEBUG_RSEQ enabled the fast path was never taken, so
nothing changes there either.

has_syscall_work() then has a single user left, so open-code it.

No functional changes intended.

Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Ada Couprie Diaz <ada.coupriediaz@xxxxxxx>
Reviewed-by: Linus Walleij <linusw@xxxxxxxxxx>
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
 arch/arm64/kernel/syscall.c | 20 +-------------------
 1 file changed, 1 insertion(+), 19 deletions(-)

diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
index f1f0b6f3e32b..3488afd45d20 100644
--- a/arch/arm64/kernel/syscall.c
+++ b/arch/arm64/kernel/syscall.c
@@ -54,11 +54,6 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
 	syscall_set_return_value(current, regs, 0, ret);
 }
 
-static inline bool has_syscall_work(unsigned long flags)
-{
-	return unlikely(flags & _TIF_SYSCALL_WORK);
-}
-
 static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 			   const syscall_fn_t syscall_table[])
 {
@@ -95,7 +90,7 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 		return;
 	}
 
-	if (has_syscall_work(flags)) {
+	if (unlikely(flags & _TIF_SYSCALL_WORK)) {
 		/*
 		 * The de-facto standard way to skip a system call using ptrace
 		 * is to set the system call to -1 (NO_SYSCALL) and set x0 to a
@@ -119,19 +114,6 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
 	}
 
 	invoke_syscall(regs, scno, sc_nr, syscall_table);
-
-	/*
-	 * The tracing status may have changed under our feet, so we have to
-	 * check again. However, if we were tracing entry, then we always trace
-	 * exit regardless, as the old entry assembly did.
-	 */
-	if (!has_syscall_work(flags) && !IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
-		flags = read_thread_flags();
-		if (unlikely(flags & _TIF_SYSCALL_EXIT_WORK) || flags & _TIF_SINGLESTEP)
-			arm64_syscall_exit_to_user_mode_work(regs);
-		return;
-	}
-
 trace_exit:
 	arm64_syscall_exit_to_user_mode_work(regs);
 }
-- 
2.34.1