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Date: Thu, 24 Sep 2026 11:18:06 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sean Christopherson <seanjc@google.com>
Cc: "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>,
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	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: <20260924091806.GK2009045@noisy.programming.kicks-ass.net>
References: <179005108298.388919.4535333252892590932.stgit@devnote2>
 <179005110742.388919.1509641807739909065.stgit@devnote2>
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In-Reply-To: <arPjK1-PVp_UqAsQ@google.com>

On Wed, Sep 23, 2026 at 07:33:15AM -0700, Sean Christopherson 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()

So I would *love* to delete KGDB support for x86. Is anybody actually
using this turd?