[PATCH 2/2] x86/hw_breakpoint: Add arch_reinstall_hw_breakpoint

[Jinchao Wang <wangjinchao600@gmail.com>]

The new `arch_reinstall_hw_breakpoint` function can be used in an
atomic context, unlike the more expensive free and re-allocation path.

This patch adds `BP_SLOT_ACTION_REINSTALL` to the `enum` and updates the
`manage_bp_slot` helper to correctly handle the action. This allows
callers to efficiently re-establish an existing breakpoint.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 arch/x86/include/asm/hw_breakpoint.h |  2 ++
 arch/x86/kernel/hw_breakpoint.c      | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
index bd437a30dbf2..d1cc6db8a59f 100644
--- a/arch/x86/include/asm/hw_breakpoint.h
+++ b/arch/x86/include/asm/hw_breakpoint.h
@@ -20,6 +20,7 @@ struct arch_hw_breakpoint {
 
 enum bp_slot_action {
 	BP_SLOT_ACTION_INSTALL,
+	BP_SLOT_ACTION_REINSTALL,
 	BP_SLOT_ACTION_UNINSTALL,
 };
 
@@ -64,6 +65,7 @@ extern int hw_breakpoint_exceptions_notify(struct notifier_block *unused,
 
 
 int arch_install_hw_breakpoint(struct perf_event *bp);
+int arch_reinstall_hw_breakpoint(struct perf_event *bp);
 void arch_uninstall_hw_breakpoint(struct perf_event *bp);
 void hw_breakpoint_pmu_read(struct perf_event *bp);
 void hw_breakpoint_pmu_unthrottle(struct perf_event *bp);
diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 1736063a82b7..99223695cee8 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -94,6 +94,10 @@ static int manage_bp_slot(struct perf_event *bp, enum bp_slot_action action)
 		old_bp = NULL;
 		new_bp = bp;
 		break;
+	case BP_SLOT_ACTION_REINSTALL:
+		old_bp = bp;
+		new_bp = bp;
+		break;
 	case BP_SLOT_ACTION_UNINSTALL:
 		old_bp = bp;
 		new_bp = NULL;
@@ -192,6 +196,23 @@ int arch_install_hw_breakpoint(struct perf_event *bp)
 	return arch_manage_bp(bp, BP_SLOT_ACTION_INSTALL);
 }
 
+/*
+ * Reinstall a hardware breakpoint on the current CPU.
+ *
+ * This function is used to re-establish a perf counter hardware breakpoint.
+ * It finds the debug address register slot previously allocated for the
+ * breakpoint and re-enables it by writing the address to the debug register
+ * and setting the corresponding bits in the debug control register (DR7).
+ *
+ * It is expected that the breakpoint's event context lock is already held
+ * and interrupts are disabled, ensuring atomicity and safety from other
+ * event handlers.
+ */
+int arch_reinstall_hw_breakpoint(struct perf_event *bp)
+{
+	return arch_manage_bp(bp, BP_SLOT_ACTION_REINSTALL);
+}
+
 /*
  * Uninstall the breakpoint contained in the given counter.
  *
-- 
2.43.0