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Subject[PATCH v2 35/36] x86/entry: Split and inline prepare_exit_to_usermode
Date
GCC is unable to properly optimize functions that have a very short
likely case and a longer and register-heavier cold part -- it fails
to sink all of the register saving and stack frame setup code into
the unlikely part.

Help it out with prepare_exit_to_usermode by splitting it into two
parts and inline the hot part.

Saves 6-8 cycles for compat syscalls.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/entry/common.c | 43 ++++++++++++++++++++++++++++---------------
1 file changed, 28 insertions(+), 15 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 3d44c83f2769..d6b8f4fd8274 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -218,14 +218,12 @@ long syscall_trace_enter(struct pt_regs *regs)
return syscall_trace_enter_phase2(regs, arch, phase1_result);
}

-/* Called with IRQs disabled. */
-__visible void prepare_exit_to_usermode(struct pt_regs *regs)
-{
- if (IS_ENABLED(CONFIG_PROVE_LOCKING) && WARN_ON(!irqs_disabled()))
- local_irq_disable();
-
- lockdep_sys_exit();
+#define EXIT_TO_USERMODE_LOOP_FLAGS \
+ (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE | \
+ _TIF_NEED_RESCHED | _TIF_USER_RETURN_NOTIFY)

+static void exit_to_usermode_loop(struct pt_regs *regs, u32 cached_flags)
+{
/*
* In order to return to user mode, we need to have IRQs off with
* none of _TIF_SIGPENDING, _TIF_NOTIFY_RESUME, _TIF_USER_RETURN_NOTIFY,
@@ -235,14 +233,6 @@ __visible void prepare_exit_to_usermode(struct pt_regs *regs)
* work to clear some of the flags can sleep.
*/
while (true) {
- u32 cached_flags =
- READ_ONCE(pt_regs_to_thread_info(regs)->flags);
-
- if (!(cached_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME |
- _TIF_UPROBE | _TIF_NEED_RESCHED |
- _TIF_USER_RETURN_NOTIFY)))
- break;
-
/* We have work to do. */
local_irq_enable();

@@ -266,7 +256,30 @@ __visible void prepare_exit_to_usermode(struct pt_regs *regs)

/* Disable IRQs and retry */
local_irq_disable();
+
+ cached_flags = READ_ONCE(pt_regs_to_thread_info(regs)->flags);
+
+ if (!(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
+ break;
+
}
+}
+
+/* Called with IRQs disabled. */
+__visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
+{
+ u32 cached_flags;
+
+ if (IS_ENABLED(CONFIG_PROVE_LOCKING) && WARN_ON(!irqs_disabled()))
+ local_irq_disable();
+
+ lockdep_sys_exit();
+
+ cached_flags =
+ READ_ONCE(pt_regs_to_thread_info(regs)->flags);
+
+ if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
+ exit_to_usermode_loop(regs, cached_flags);

user_enter();
}
--
2.4.3


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