On Wed, Aug 19, 2015 at 05:03:20PM +0100, Catalin Marinas wrote:
> On Wed, Aug 19, 2015 at 04:09:49PM +0100, Will Deacon wrote:
> > @@ -613,13 +609,14 @@ ENDPROC(cpu_switch_to)
> > */
> > ret_fast_syscall:
> > disable_irq // disable interrupts
> > + str x0, [sp, #S_X0] // returned x0
> > ldr x1, [tsk, #TI_FLAGS] // re-check for syscall tracing
> > and x2, x1, #_TIF_SYSCALL_WORK
> > cbnz x2, ret_fast_syscall_trace
> > and x2, x1, #_TIF_WORK_MASK
> > - cbnz x2, fast_work_pending
> > + cbnz x2, work_pending
> > enable_step_tsk x1, x2
> > - kernel_exit 0, ret = 1
> > + kernel_exit 0
> > ret_fast_syscall_trace:
> > enable_irq // enable interrupts
> > b __sys_trace_return
>
> There is another str x0 in __sys_trace_return which I think we could
> remove.
Hmm, I don't think we can remove that. It's needed on the slowpath to
update the pt_regs with either -ENOSYS (for __ni_sys_trace) or the
syscall return value from the blr in __sys_trace.
What we can do instead is change the branch above to branch to
__sys_trace_return_skipped. Patch below.
Will
--->8
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 2a5e64ccc991..088322ff1ba0 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -619,7 +619,7 @@ ret_fast_syscall:
kernel_exit 0
ret_fast_syscall_trace:
enable_irq // enable interrupts
- b __sys_trace_return
+ b __sys_trace_return_skipped // we already saved x0
/*
* Ok, we need to do extra processing, enter the slow path.
--
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