linux-kernel - Re: [patch 4/8] x86/tsc: Verify TSC

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Date:   Mon, 21 Nov 2016 14:57:13 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [patch 4/8] x86/tsc: Verify TSC_ADJUST from idle

Thomas Gleixner <tglx@...utronix.de> writes:

> 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.

idle entry is a time critical code path too, because idle periods are
often very short.  rdmsr is a us+ at least. You'll likely make
workloads that do a lot of short sleeps noticeable slower.

If anything load limit it aggressively.

BTW I don't think this situation of SMM tampering with TSC is common
anyways, so it may be simply not worth checking, or perhaps
only with a debug boot option.

-Andi