On Wed, Aug 19, 2015 at 05:35:05PM +0100, Catalin Marinas wrote:

On Wed, Aug 19, 2015 at 05:23:59PM +0100, Will Deacon wrote:

quoted

On Wed, Aug 19, 2015 at 05:03:20PM +0100, Catalin Marinas wrote:

quoted

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

That would do. With this added:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

(or I can take it for 4.2 but I'd like more testing like LTP)

Yeah, I'll run some tests overnight and see how it holds up.

Will