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Date: Thu, 24 Sep 2026 10:31:40 +0900
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>, "Masami Hiramatsu (Google)"
 <mhiramat@kernel.org>, Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar
 <mingo@kernel.org>, Jinchao Wang <wangjinchao600@gmail.com>, Mathieu
 Desnoyers <mathieu.desnoyers@efficios.com>, Thomas Gleixner
 <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>, Dave Hansen
 <dave.hansen@linux.intel.com>, "H . Peter Anvin" <hpa@zytor.com>, Alexander
 Shishkin <alexander.shishkin@linux.intel.com>, Ian Rogers
 <irogers@google.com>, linux-kernel@vger.kernel.org,
 linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
 linux-perf-users@vger.kernel.org, x86@kernel.org, Paolo Bonzini
 <pbonzini@redhat.com>, kvm@vger.kernel.org
Subject: Re: [PATCH v17 02/13] perf/x86, KVM: Prevent host debug register
 leak into guest OS on NMI
Message-Id: <20260924103140.0a40bb18812cbb913be5e662@kernel.org>
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On Wed, 23 Sep 2026 07:33:15 -0700
Sean Christopherson <seanjc@google.com> wrote:

> 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@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) <mhiramat@kernel.org>