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

On Mon, 3 Aug 2026 08:57:44 +0200
Peter Zijlstra <[email protected]> wrote:

> On Sun, Aug 02, 2026 at 05:18:37PM +0900, Masami Hiramatsu (Google) wrote:
> > From: Jinchao Wang <[email protected]>
> > 
> > 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'.

Hmm, I think even if so, this detection is better to be handled in
hw_breakpoint layer. And I plan to use this from kprobe events,
which is also a kind of NMI.

> 
> > 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?

nmi_dr7_seq is for sequence number of operation, which is for
detecting dr7 overwrite in NMI.
nmi_dr7 is for saving the DR7.

Thanks,


-- 
Masami Hiramatsu (Google) <[email protected]>