Re: [PATCH v3 01/19] x86/hw_breakpoint: introduce arch_reinstall_hw_breakpoint() for atomic context

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

Hi Jinchao,

On Wed, 10 Sep 2025 13:23:10 +0800
Jinchao Wang <wangjinchao600@gmail.com> wrote:

> Introduce arch_reinstall_hw_breakpoint() to update hardware breakpoint
> parameters (address, length, type) without freeing and reallocating the
> debug register slot.
> 
> This allows atomic updates in contexts where memory allocation is not
> permitted, such as kprobe handlers.
> 
> Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
> ---
>  arch/x86/include/asm/hw_breakpoint.h |  1 +
>  arch/x86/kernel/hw_breakpoint.c      | 50 ++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+)
> 
> diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
> index 0bc931cd0698..bb7c70ad22fe 100644
> --- a/arch/x86/include/asm/hw_breakpoint.h
> +++ b/arch/x86/include/asm/hw_breakpoint.h
> @@ -59,6 +59,7 @@ 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 b01644c949b2..89135229ed21 100644
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -132,6 +132,56 @@ int arch_install_hw_breakpoint(struct perf_event *bp)
>  	return 0;
>  }
>  
> +/*
> + * 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)
> +{
> +	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
> +	unsigned long *dr7;
> +	int i;
> +
> +	lockdep_assert_irqs_disabled();
> +
> +	for (i = 0; i < HBP_NUM; i++) {
> +		struct perf_event **slot = this_cpu_ptr(&bp_per_reg[i]);
> +
> +		if (*slot == bp)
> +			break;
> +	}
> +
> +	if (WARN_ONCE(i == HBP_NUM, "Can't find a matching breakpoint slot"))
> +		return -EINVAL;
> +
> +	set_debugreg(info->address, i);
> +	__this_cpu_write(cpu_debugreg[i], info->address);
> +
> +	dr7 = this_cpu_ptr(&cpu_dr7);
> +	*dr7 |= encode_dr7(i, info->len, info->type);
> +
> +	/*
> +	 * Ensure we first write cpu_dr7 before we set the DR7 register.
> +	 * This ensures an NMI never see cpu_dr7 0 when DR7 is not.
> +	 */
> +	barrier();
> +
> +	set_debugreg(*dr7, 7);
> +	if (info->mask)
> +		amd_set_dr_addr_mask(info->mask, i);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(arch_reinstall_hw_breakpoint);

Please do not expose the arch dependent symbol. Instead, you should
expose an arch independent wrapper.

Anyway, you also need to share the same code with arch_install_hw_breakpoint()
like below;

Thanks,


diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 89135229ed21..2f3c5406999e 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -84,6 +84,28 @@ int decode_dr7(unsigned long dr7, int bpnum, unsigned *len, unsigned *type)
 	return (dr7 >> (bpnum * DR_ENABLE_SIZE)) & 0x3;
 }
 
+static void __arch_install_hw_breakpoint(struct perf_event *bp, int regno)
+{
+	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
+	unsigned long *dr7;
+
+	set_debugreg(info->address, regno);
+	__this_cpu_write(cpu_debugreg[i], info->address);
+
+	dr7 = this_cpu_ptr(&cpu_dr7);
+	*dr7 |= encode_dr7(i, info->len, info->type);
+
+	/*
+	 * Ensure we first write cpu_dr7 before we set the DR7 register.
+	 * This ensures an NMI never see cpu_dr7 0 when DR7 is not.
+	 */
+	barrier();
+
+	set_debugreg(*dr7, 7);
+	if (info->mask)
+		amd_set_dr_addr_mask(info->mask, i);
+}
+
 /*
  * Install a perf counter breakpoint.
  *
@@ -95,8 +117,6 @@ int decode_dr7(unsigned long dr7, int bpnum, unsigned *len, unsigned *type)
  */
 int arch_install_hw_breakpoint(struct perf_event *bp)
 {
-	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
-	unsigned long *dr7;
 	int i;
 
 	lockdep_assert_irqs_disabled();
@@ -113,22 +133,7 @@ int arch_install_hw_breakpoint(struct perf_event *bp)
 	if (WARN_ONCE(i == HBP_NUM, "Can't find any breakpoint slot"))
 		return -EBUSY;
 
-	set_debugreg(info->address, i);
-	__this_cpu_write(cpu_debugreg[i], info->address);
-
-	dr7 = this_cpu_ptr(&cpu_dr7);
-	*dr7 |= encode_dr7(i, info->len, info->type);
-
-	/*
-	 * Ensure we first write cpu_dr7 before we set the DR7 register.
-	 * This ensures an NMI never see cpu_dr7 0 when DR7 is not.
-	 */
-	barrier();
-
-	set_debugreg(*dr7, 7);
-	if (info->mask)
-		amd_set_dr_addr_mask(info->mask, i);
-
+	__arch_install_hw_breakpoint(bp, i);
 	return 0;
 }
 
@@ -146,8 +151,6 @@ int arch_install_hw_breakpoint(struct perf_event *bp)
  */
 int arch_reinstall_hw_breakpoint(struct perf_event *bp)
 {
-	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
-	unsigned long *dr7;
 	int i;
 
 	lockdep_assert_irqs_disabled();
@@ -162,22 +165,7 @@ int arch_reinstall_hw_breakpoint(struct perf_event *bp)
 	if (WARN_ONCE(i == HBP_NUM, "Can't find a matching breakpoint slot"))
 		return -EINVAL;
 
-	set_debugreg(info->address, i);
-	__this_cpu_write(cpu_debugreg[i], info->address);
-
-	dr7 = this_cpu_ptr(&cpu_dr7);
-	*dr7 |= encode_dr7(i, info->len, info->type);
-
-	/*
-	 * Ensure we first write cpu_dr7 before we set the DR7 register.
-	 * This ensures an NMI never see cpu_dr7 0 when DR7 is not.
-	 */
-	barrier();
-
-	set_debugreg(*dr7, 7);
-	if (info->mask)
-		amd_set_dr_addr_mask(info->mask, i);
-
+	__arch_install_hw_breakpoint(bp, i);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(arch_reinstall_hw_breakpoint);

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>