Re: [PATCH 1/2] x86/hw_breakpoint: Unify breakpoint install/uninstall

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

On Wed, 10 Sep 2025 17:39:34 +0800
Jinchao Wang <wangjinchao600@gmail.com> wrote:

> Consolidate breakpoint management into a single helper function to
> reduce code duplication. This introduces new static helpers for
> slot management and debug register manipulation.
> 
> Also, add `<linux/types.h>` to the header file to fix a build
> dependency.

Looks good to me. Just some nitpicks.

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

> 
> Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
> ---
>  arch/x86/include/asm/hw_breakpoint.h |   7 +-
>  arch/x86/kernel/hw_breakpoint.c      | 151 ++++++++++++++++-----------
>  2 files changed, 96 insertions(+), 62 deletions(-)
> 
> diff --git a/arch/x86/include/asm/hw_breakpoint.h b/arch/x86/include/asm/hw_breakpoint.h
> index 0bc931cd0698..bd437a30dbf2 100644
> --- a/arch/x86/include/asm/hw_breakpoint.h
> +++ b/arch/x86/include/asm/hw_breakpoint.h
> @@ -3,8 +3,8 @@
>  #define	_I386_HW_BREAKPOINT_H
>  
>  #include <uapi/asm/hw_breakpoint.h>
> -

nit: Why this line is removed?

>  #define	__ARCH_HW_BREAKPOINT_H
> +#include <linux/types.h>
>  
>  /*
>   * The name should probably be something dealt in
> @@ -18,6 +18,11 @@ struct arch_hw_breakpoint {
>  	u8		type;
>  };
>  
> +enum bp_slot_action {
> +	BP_SLOT_ACTION_INSTALL,
> +	BP_SLOT_ACTION_UNINSTALL,
> +};
> +
>  #include <linux/kdebug.h>
>  #include <linux/percpu.h>
>  #include <linux/list.h>
> diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
> index b01644c949b2..1736063a82b7 100644
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -48,7 +48,6 @@ static DEFINE_PER_CPU(unsigned long, cpu_debugreg[HBP_NUM]);
>   */
>  static DEFINE_PER_CPU(struct perf_event *, bp_per_reg[HBP_NUM]);
>  
> -

Ditto.

>  static inline unsigned long
>  __encode_dr7(int drnum, unsigned int len, unsigned int type)
>  {
> @@ -84,54 +83,115 @@ int decode_dr7(unsigned long dr7, int bpnum, unsigned *len, unsigned *type)
>  	return (dr7 >> (bpnum * DR_ENABLE_SIZE)) & 0x3;
>  }
>  
> -/*
> - * Install a perf counter breakpoint.
> - *
> - * We seek a free debug address register and use it for this
> - * breakpoint. Eventually we enable it in the debug control register.
> - *
> - * Atomic: we hold the counter->ctx->lock and we only handle variables
> - * and registers local to this cpu.
> - */
> -int arch_install_hw_breakpoint(struct perf_event *bp)
> +static int manage_bp_slot(struct perf_event *bp, enum bp_slot_action action)
>  {
> -	struct arch_hw_breakpoint *info = counter_arch_bp(bp);
> -	unsigned long *dr7;
> -	int i;
> +	struct perf_event *old_bp;
> +	struct perf_event *new_bp;
> +	int slot;
> +
> +	switch (action) {
> +	case BP_SLOT_ACTION_INSTALL:
> +		old_bp = NULL;
> +		new_bp = bp;
> +		break;
> +	case BP_SLOT_ACTION_UNINSTALL:
> +		old_bp = bp;
> +		new_bp = NULL;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
>  
>  	lockdep_assert_irqs_disabled();
>  
> -	for (i = 0; i < HBP_NUM; i++) {
> -		struct perf_event **slot = this_cpu_ptr(&bp_per_reg[i]);
> +	for (slot = 0; slot < HBP_NUM; slot++) {
> +		struct perf_event **curr = this_cpu_ptr(&bp_per_reg[slot]);
>  
> -		if (!*slot) {
> -			*slot = bp;
> -			break;
> +		if (*curr == old_bp) {
> +			*curr = new_bp;
> +			return slot;
>  		}
>  	}
>  
> -	if (WARN_ONCE(i == HBP_NUM, "Can't find any breakpoint slot"))
> -		return -EBUSY;
> +	if (old_bp) {
> +		WARN_ONCE(1, "Can't find matching breakpoint slot");
> +		return -EINVAL;
> +	}
>  
> -	set_debugreg(info->address, i);
> -	__this_cpu_write(cpu_debugreg[i], info->address);
> +	WARN_ONCE(1, "No free breakpoint slots");
> +	return -EBUSY;
> +}
>  
> -	dr7 = this_cpu_ptr(&cpu_dr7);
> -	*dr7 |= encode_dr7(i, info->len, info->type);
> +static void setup_hwbp(struct arch_hw_breakpoint *info, int slot, bool enable)
> +{
> +	unsigned long dr7;
> +
> +	set_debugreg(info->address, slot);
> +	__this_cpu_write(cpu_debugreg[slot], info->address);
> +
> +	dr7 = this_cpu_read(cpu_dr7);
> +	if (enable)
> +		dr7 |= encode_dr7(slot, info->len, info->type);
> +	else
> +		dr7 &= ~__encode_dr7(slot, 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.
> +	 * Enabling:
> +	 *   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.
>  	 */
> +	if (enable)
> +		this_cpu_write(cpu_dr7, dr7);
> +
>  	barrier();
>  
> -	set_debugreg(*dr7, 7);
> +	set_debugreg(dr7, 7);
> +
>  	if (info->mask)
> -		amd_set_dr_addr_mask(info->mask, i);
> +		amd_set_dr_addr_mask(enable ? info->mask : 0, slot);
> +
> +	/*
> +	 * Disabling:
> +	 *   Ensure the write to cpu_dr7 is after we've set the DR7 register.
> +	 *   This ensures an NMI never see cpu_dr7 0 when DR7 is not.
> +	 */
> +	if (!enable)
> +		this_cpu_write(cpu_dr7, dr7);
> +}
> +
> +static int arch_manage_bp(struct perf_event *bp, enum bp_slot_action action)
> +{
> +	struct arch_hw_breakpoint *info;
> +	bool install = true;
> +	int slot;
> +
> +	if (action == BP_SLOT_ACTION_UNINSTALL)
> +		install = false;

This looks a bit unnecessary. 

> +
> +	slot = manage_bp_slot(bp, action);
> +	if (slot < 0)
> +		return slot;
> +
> +	info = counter_arch_bp(bp);
> +	setup_hwbp(info, slot, install);

since you can do

	setup_hwbp(info, slot,
		   action != BP_SLOT_ACTION_UNINSTALL);

Thank you,


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