Re: [PATCH v3 00/13] arm64: debug: remove hook registration, split exception entry

On Mon, Jun 09, 2025 at 06:34:00PM +0100, Ada Couprie Diaz wrote:

Hi,

This series simplifies the debug exception entry path by removing handler
registration mechanisms for the debug exception handlers, a holdover from
the arm kernel, as well as the break and stepping handlers.
This moves much of the code related to debug exceptions outside of
`mm/fault.c` where it didn't make much sense.
This allows us to split the debug exception entries: going from one common
path per EL for all debug exceptions to a unique one per exception and EL.

The result is a much simpler and fully static exception entry path, which
we tailor to the different exceptions and their constraints.

The series is structured as such :
1 : related clean-up in the signle step handler
2 : related refactor of `aarch32_break_handler()`
3-5 : software breakpoints and single step handlers registration removal
6: preparatory function move that is made internal in patch 13
8: preparatory refactor of `reinstall_suspended_bps()`
7, 9-13 : debug exception splitting and handler registration removal

* Patch 3 copies and extends the `early_brk64` break handling for the
normal path, byassing the dynamic registration.
* Patch 4 does something similar for the single stepping handlers.
* Patches 7, and 9 through 13 split each individual debug exception from
the unified path by creating specific handlers, adapting them to
their constraints and calling into them, bypassing the dynamically
registered handlers used before.
* Patch 8 refactors `reinstall_suspended_bps()` in preparation for the
single-step exception entry split, as it is moved out of the handler.
This could be squashed in patch 9 (the single-step exception split), but
I opted to separate it for clarity (and because the commit message
for patch 9 is already 45 lines long !).
* Patches 5 and 13 are clean-ups removing the code that has been replaced
and made redundant by the preceding patches.

PREEMPT_RT
===

Of note, this allows us to make the single exception handling mostly
preemptible coming from EL0 in patch 9, fixing an issue with PREEMPT_RT[0].
The commit message details the reasoning on why this should be safe.
It is *definitely* not preemptible at EL1 in the current state, given
that the EL1 software step exception is handled by KGDB.

CC-ing Luis as he did a lot of testing on v2 and originally
reported the issue.

Cc: "Luis Claudio R. Goncalves" <redacted>

We can do the same for the software break exception coming from EL0,
which works in a very similar way.

However, the hardware breakpoint and watchpoint exceptions also have
a sleeping while non-preemptible issue with PREEMPT_RT, but this is
much trickier to fix, so this won't be fixed in this series.
A bit more details in [1].

I hesitated to add comments within the BRK64 and single-step handlers,
on the EL0 paths, to highlight that they would be preemptible and to be
wary if modifying or adding to them, but I opted not to in the end.
(Partly as I didn't find a satisfactory way to do it !)
I would be happy to add some if anyone finds it would be beneficial.

Tested-by: Luis Claudio R. Goncalves <redacted>

Works as advertised, fixes the reported RT problem, fails on the specific
case Ada described above. :)

Best regards,
Luis