arm64: factor work_pending state machine to C

Currently ret_fast_syscall, work_pending, and ret_to_user form an ad-hoc
state machine that can be difficult to reason about due to duplicated
code and a large number of branch targets.

This patch factors the common logic out into the existing
do_notify_resume function, converting the code to C in the process,
making the code more legible.

This patch tries to closely mirror the existing behaviour while using
the usual C control flow primitives. As local_irq_{disable,enable} may
be instrumented, we balance exception entry (where we will almost most
likely enable IRQs) with a call to trace_hardirqs_on just before the
return to userspace.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 4 insertions, 8 deletions

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 441420ca7d08..6a64182822e5 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -707,18 +707,13 @@ ret_fast_syscall_trace:
  * Ok, we need to do extra processing, enter the slow path.
  */
 work_pending:
-        tbnz    x1, #TIF_NEED_RESCHED, work_resched
-        /* TIF_SIGPENDING, TIF_NOTIFY_RESUME or TIF_FOREIGN_FPSTATE case */
         mov     x0, sp                          // 'regs'
-        enable_irq                              // enable interrupts for do_notify_resume()
         bl      do_notify_resume
-        b       ret_to_user
-work_resched:
 #ifdef CONFIG_TRACE_IRQFLAGS
-        bl      trace_hardirqs_off              // the IRQs are off here, inform the tracing code
+        bl      trace_hardirqs_on               // enabled while in userspace
 #endif
-        bl      schedule
-
+        ldr     x1, [tsk, #TI_FLAGS]            // re-check for single-step
+        b       finish_ret_to_user
 /*
  * "slow" syscall return path.
  */
@@ -727,6 +722,7 @@ ret_to_user:
         ldr     x1, [tsk, #TI_FLAGS]
         and     x2, x1, #_TIF_WORK_MASK
         cbnz    x2, work_pending
+finish_ret_to_user:
         enable_step_tsk x1, x2
         kernel_exit 0
 ENDPROC(ret_to_user)