Re: [PATCH v2 06/18] perf/hw_breakpoint: add arch-independent hw_breakpoint_modify_local()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 04, 2025 at 08:21:03AM +0800, Jinchao Wang wrote:
> Introduce hw_breakpoint_modify_local() as a generic helper to modify an
> existing hardware breakpoint. The function invokes
> hw_breakpoint_arch_parse() and delegates the reinstall step to the
> architecture via arch_reinstall_hw_breakpoint().
> 
> A weak default implementation of arch_reinstall_hw_breakpoint() is
> provided, returning -EOPNOTSUPP on architectures without support.
> 
> This makes the interface arch-independent while allowing x86 (and others)
> to provide their own implementation.
> 
> Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
> ---
>  include/linux/hw_breakpoint.h |  1 +
>  kernel/events/hw_breakpoint.c | 18 ++++++++++++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
> index db199d653dd1..9453b5bdb443 100644
> --- a/include/linux/hw_breakpoint.h
> +++ b/include/linux/hw_breakpoint.h
> @@ -67,6 +67,7 @@ extern int
>  modify_user_hw_breakpoint_check(struct perf_event *bp, struct perf_event_attr *attr,
>  				bool check);
>  
> +int hw_breakpoint_modify_local(struct perf_event *bp, struct perf_event_attr *attr);
>  /*
>   * Kernel breakpoints are not associated with any particular thread.
>   */
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 8ec2cb688903..ff428739f71e 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -983,6 +983,24 @@ static void hw_breakpoint_del(struct perf_event *bp, int flags)
>  	arch_uninstall_hw_breakpoint(bp);
>  }
>  
> +int hw_breakpoint_modify_local(struct perf_event *bp, struct perf_event_attr *attr)
> +{
> +	int err;
> +
> +	err = hw_breakpoint_arch_parse(bp, attr, counter_arch_bp(bp));
> +	if (err)
> +		return err;
> +
> +	return arch_reinstall_hw_breakpoint(bp);
> +}
> +EXPORT_SYMBOL(hw_breakpoint_modify_local);
> +
> +/* weak fallback for arches without support */
> +__weak int arch_reinstall_hw_breakpoint(struct perf_event *bp)
> +{
> +	return -EOPNOTSUPP;
> +}

Again, so much fail :/

So we have:

{register,modify,unregister}_user_hw_breakpoint()

and

{register,unregister}_wide_hw_breakpoint()

And you choose to extend this latter with hw_breakpoint_modify_local()
instead of sticking with the naming scheme and say adding:

modify_wide_hw_breakpoint_local().

Also, again, that EXPORT is a fail, these other interfaces are all
EXPORT_SYMBOL_GPL().

Also note that modify_user_hw_breakpoint() doesn't seem to need new arch
hooks. Yet you fail to explain why you think you do.