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Date: Wed, 23 Sep 2026 10:51:39 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	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: <20260923085139.GV776954@noisy.programming.kicks-ass.net>
References: <179005108298.388919.4535333252892590932.stgit@devnote2>
 <179005110742.388919.1509641807739909065.stgit@devnote2>
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In-Reply-To: <179005110742.388919.1509641807739909065.stgit@devnote2>

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.