[PATCH 2/3] x86/hw_breakpoints: Add arch_modify_local_hw_breakpoint_addr() API

[Jinchao Wang]

Wprobe needs to move an active per-CPU watchpoint without releasing and
reserving its hardware slot.

Add arch_modify_local_hw_breakpoint_addr() to find the installed local
slot and update only its address shadow and hardware debug address
register. Publish the shadow first so hw_breakpoint_restore() observes
the new address if an NMI interrupts the update.

The caller must provide an installed local event and a valid address.
Slot ownership, breakpoint type, length, mask and DR7 remain unchanged.

Signed-off-by: Jinchao Wang <wangjinchao600@xxxxxxxxx>
---
 arch/x86/include/asm/hw_breakpoint.h |  2 ++
 arch/x86/kernel/hw_breakpoint.c      | 20 ++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
index 0bc931cd0698..a1ecce536074 100644
--- a/arch/x86/include/asm/hw_breakpoint.h
+++ b/arch/x86/include/asm/hw_breakpoint.h
@@ -59,6 +59,8 @@ extern int hw_breakpoint_exceptions_notify(struct notifier_block *unused,
 
 
 int arch_install_hw_breakpoint(struct perf_event *bp);
+void arch_modify_local_hw_breakpoint_addr(struct perf_event *bp,
+					  unsigned long addr);
 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 9ef24b55737f..ff1b9b78f78c 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -128,6 +128,26 @@ int arch_install_hw_breakpoint(struct perf_event *bp)
 	return 0;
 }
 
+void arch_modify_local_hw_breakpoint_addr(struct perf_event *bp,
+					  unsigned long addr)
+{
+	int i;
+
+	lockdep_assert_irqs_disabled();
+
+	for (i = 0; i < HBP_NUM; i++) {
+		if (this_cpu_read(bp_per_reg[i]) == bp)
+			break;
+	}
+
+	if (WARN_ONCE(i == HBP_NUM, "Can't find any breakpoint slot"))
+		return;
+
+	this_cpu_write(cpu_debugreg[i], addr);
+	barrier();
+	set_debugreg(addr, i);
+}
+
 /*
  * Uninstall the breakpoint contained in the given counter.
  *
-- 
2.53.0