Re: [PATCH v17 02/13] perf/x86, KVM: Prevent host debug register leak into guest OS on NMI

[Sean Christopherson]

On Wed, Sep 23, 2026, Peter Zijlstra wrote:
> On Tue, Sep 22, 2026 at 01:25:07PM +0900, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > 
> > When KVM enters a guest OS, host hardware breakpoints are disabled
> > before running the guest. However, an NMI can occur while executing
> > in guest mode or during guest transition, where
> > arch_install_hw_breakpoint() can be invoked from NMI context.
> 
> How? AFAICT perf doesn't do that. arch_install_hw_breakpoint() is called
> through pmu::add() which is strictly IPI context.

I assume it's:

  kgdb_nmi_handler()
  |
  -> kgdb_nmicallin()
     |
     -> kgdb_cpu_enter()
        |
        -> arch_kgdb_ops.correct_hw_break()
           |
           -> kgdb_correct_hw_break()
              |
              -> arch_install_hw_breakpoint()