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Date: Wed, 23 Sep 2026 11:15:55 +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,
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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: <20260923091555.GX776954@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:
> diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
> index f846c15f21ca..0473a5c95856 100644
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -102,6 +102,9 @@ int arch_install_hw_breakpoint(struct perf_event *bp)
>  
>  	lockdep_assert_irqs_disabled();
>  
> +	if (perf_guest_in_guest())

That naming is hilariously bad :-)

> +		return -EBUSY;
> +
>  	for (i = 0; i < HBP_NUM; i++) {
>  		struct perf_event **slot = this_cpu_ptr(&bp_per_reg[i]);
>  

Note how the other -EBUSY return is a WARN. Why is silently not doing
anything not a WARN in this case?

And I suppose this is KGDB again?