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Date: Sun, 20 Nov 2016 14:10:02 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        x86@kernel.org, Borislav Petkov <bp@alien8.de>,
        Yinghai Lu <yinghai@kernel.org>
Subject: Re: [patch 4/8] x86/tsc: Verify TSC_ADJUST from idle
Message-ID: <20161120131002.GZ3117@twins.programming.kicks-ass.net>
References: <20161119133816.633700010@linutronix.de>
 <20161119134017.732180441@linutronix.de>
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On Sat, Nov 19, 2016 at 01:47:37PM -0000, Thomas Gleixner wrote:
> When entering idle, it's a good oportunity to verify that the TSC_ADJUST
> MSR has not been tampered with (BIOS hiding SMM cycles). If tampering is
> detected, emit a warning and restore it to the previous value.

> +++ b/arch/x86/kernel/process.c
> @@ -277,6 +277,7 @@ void exit_idle(void)
>  
>  void arch_cpu_idle_enter(void)
>  {
> +	tsc_verify_tsc_adjust();
>  	local_touch_nmi();
>  	enter_idle();
>  }

Doing a RDMSR on the idle path isn't going to be popular. That path is
already way too slow.