[PATCH 0/2] arm64/debug: clean up some HW BP edge cases

[Will Deacon]

On Wed, May 27, 2026 at 05:15:51PM +0100, Ada Couprie Diaz wrote:
> This is a small series focused on fixing some unsoundess and edge case
> related to hardware breakpoints and watchpoints.
> 
> It is motivated in part by the recent discussion on the
> FEAT_Debugv8p6 series[0][1] regarding unclear guarantees on interruptions.
> 
> It doesn't address the issue of instrumentation, as NO_KPROBE is not
> sufficient to prevent it (specifically ftrace) and would require a pass
> over the whole debug handling code.
> 
> The bugs themselves are very much of the self-inflicted
> "you should not be doing this" kind and not urgent, but it does solve
> some of the questions on the FEAT_Dbugv8p6 series.
> 
> Most of the raw additions are comments trying to give more context to
> understand `hw_breakpoint_thread_switch()`.

Hmm. From what I can tell, these patches are trying to handle various
cases where we take a debug exception while executing the hw_breakpoint
code itself. Even with these two fixes, is perf expecting to handle that
sort of thing? I think it would be a lot more robust if we just prevented
this from happening in the first place, similarly to the discussion from
the other day around kprobes [2].

Will

[2] https://lore.kernel.org/all/alpuL10h7-OK2hFb@willie-the-truck/