Re: [PATCH v11 01/11] x86/hw_breakpoints: Make DR7 updates NMI safe

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Aug 02, 2026 at 05:18:37PM +0900, Masami Hiramatsu (Google) wrote:
> From: Jinchao Wang <wangjinchao600@gmail.com>
> 
> Hardware breakpoint installation and removal run with IRQs disabled, but
> an NMI can still enter the same code through KGDB. The interrupted
> operation and the NMI can consequently claim the same slot or overwrite
> each other's DR7 state.

Is KGDB really the only way to trip this? Mostly I think we let KGDB
have the pieces if it does something 'funny'.

> diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
> index f846c15f21ca..9ef24b55737f 100644
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -40,6 +40,9 @@
>  DEFINE_PER_CPU(unsigned long, cpu_dr7);
>  EXPORT_PER_CPU_SYMBOL(cpu_dr7);
>  
> +/* Sequence number of the per-CPU DR7 state. */
> +DEFINE_PER_CPU(unsigned int, cpu_dr7_seq);

> diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> index 3c9f60d6ca5a..f55a0cbd5927 100644
> --- a/arch/x86/kernel/nmi.c
> +++ b/arch/x86/kernel/nmi.c
> @@ -532,10 +532,13 @@ enum nmi_states {
>  static DEFINE_PER_CPU(enum nmi_states, nmi_state);
>  static DEFINE_PER_CPU(unsigned long, nmi_cr2);
>  static DEFINE_PER_CPU(unsigned long, nmi_dr7);
> +static DEFINE_PER_CPU(unsigned int, nmi_dr7_seq);

This is weird, why have two distinct sequence numbers for dr7?