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

On Thu, Sep 04, 2025 at 08:44:48AM +0200, Peter Zijlstra wrote:

On Thu, Sep 04, 2025 at 08:21:03AM +0800, Jinchao Wang wrote:

quoted

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 <redacted>
---
 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().

Thanks for your patience.

I was misled by another family:
  - hw_breakpoint_add
  - hw_breakpoint_del
  - hw_breakpoint_start
  - hw_breakpoint_stop

Since this logic was also added in the wprobe series by Masami, I will
adopt his version instead.

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.

Thanks for feedback, I will study existing code to better handle the arch
dependencies.