Re: [PATCH v3 0/5] ARM64: disable irq between breakpoint and step exception

From: Pratyush Anand
Date: Tue Aug 01 2017 - 04:32:16 EST

Hi Takahiro,

On Tuesday 01 August 2017 01:44 PM, AKASHI Takahiro wrote:
Hi Pratyush,

On Mon, Jul 31, 2017 at 04:10:28PM +0530, Pratyush Anand wrote:
v2 -> v3
- Moved step_needed from uapi structure to kernel only structure
- Re-enable interrupt if stepped instruction faults
- Modified register_wide_hw_breakpoint() to accept step_needed arg
v2 was here:

v1 -> v2:
- patch 1 of v1 has been modified to patch 1-3 of v2.
- Introduced a new event attribute step_needed and implemented
hw_breakpoint_needs_single_step() (patch 1)
- Replaced usage of is_default_overflow_handler() with
hw_breakpoint_needs_single_step(). (patch 2)
- Modified sample test to set set step_needed bit field (patch 3)
v1 was here:

samples/hw_breakpoint/data_breakpoint.c passes with x86_64 but fails with
ARM64. Even though it has been NAKed previously on upstream [1, 2], I have
tried to come up with patches which can resolve it for ARM64 as well.

I noticed that even perf step exception can go into an infinite loop if CPU
receives an interrupt while executing breakpoint/watchpoint handler. So,
event though we are not concerned about above test, we will have to find a
solution for the perf issue.

This patchset attempts to resolve both the issue. Please review.
Since, it also takes care of SW breakpoint, so I hope kgdb should also be
fine. However, I have not tested that.
@Takahiro: Will it be possible to test these patches for kgdb.

I have not yet understood the details of your patch, but
I gave it a try and didn't see any difference around the behavior
of kgdb's single stepping.

I also gave a try to James' patch, but again nothing different
as long as kgdb is concerned.
(I'm tackling some issue in single stepping at irq's kernel_exit,
in particular, 'eret'.)

You mean that you were expecting an step exception after eret (and this eret was being called from kgdb breakpoint exception handler), but you got irq exception? This is what I understood from your previous patch [0].

If that was the case, then I was expecting that this patch series should help.
See, patch 4/5:
- kgdb breakpoint handler kgdb_brk_fn() will be called from arch/arm64/kernel/debug-monitors.c: brk_handler().
- If we are expecting a step exception after servicing this breakpoint handler, then kgdb code would have called kernel_enable_single_step(). So, we should see kernel_active_single_step() true in brk_handler().
- If above happens then do_debug_exception() will make sure that PSR I bit is set before eret is called and we should not see an IRQ exception after eret.

Can you please help me with your reproducer test case?