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

[Jinchao Wang <wangjinchao600@gmail.com>]

On 9/11/25 08:46, Masami Hiramatsu (Google) wrote:
> 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,

You are right. The arch-dependent symbol has been removed and the code 
is shared

with arch_install_hw_breakpoint() in the next version of the patch.

https://lore.kernel.org/lkml/20250910093951.1330637-1-wangjinchao600@gmail.com


>
> 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);
>
-- 
Thanks,
Jinchao