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

On Wed, 23 Sep 2026 07:33:15 -0700
Sean Christopherson [off-list ref] wrote:

On Wed, Sep 23, 2026, Peter Zijlstra wrote:

quoted

On Tue, Sep 22, 2026 at 01:25:07PM +0900, Masami Hiramatsu (Google) wrote:

quoted

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

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()

Yes, it is done by KGDB. perf has no problem with it.

Thank you,

-- 
Masami Hiramatsu (Google) [off-list ref]