Re: [PATCH v2 05/18] x86/hw_breakpoint: introduce arch_reinstall_hw_breakpoint() for atomic context

Masami Hiramatsu
2025-09-09T08:00:26Z

On Mon, 8 Sep 2025 13:20:51 +0800
Jinchao Wang <wangjinchao600@gmail.com> wrote:

> On Thu, Sep 04, 2025 at 08:38:32AM +0200, Peter Zijlstra wrote:
> > On Thu, Sep 04, 2025 at 08:21:02AM +0800, Jinchao Wang 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);
> >
> > Yeah, I think not. For one this is an almost verbatim copy of
> > arch_install_hw_breakpoint() with zero re-use. Surely you've been taught
> > better?
> I introduced this to modify bp_addr in atomic context in my RFC series.
> I thought it was clearer to split the introduction and refactor.
> And then It was used in the wprobe series, so I left it as introduced
> in the RFC series.
>
> I agree your suggestion is right. I am willing to refactor after wprobe.

I'm OK to refactor it to reuse arch_install_hw_breakpoint().
My point is to have CONFIG_HAVE_REINSTALL_HW_BREAKPOINT so that
we can easily implement the dependency for other features which
requires this feature in Kconfig level.

> > And why would we want to export guts like this?
> I wanted to introduce a real-time stack corruption debugging tool,
> which needs a helper to change bp_addr in atomic context (kprobe handler).
> And wprobe needs it also.

I agree with Peter, it should not expose the architecture
dependent code directly. Instead, we need a wrapper.

Thank you,

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