Re: [patch 4/8] x86/tsc: Verify TSC_ADJUST from idle

From: Andi Kleen
Date: Mon Nov 21 2016 - 17:57:17 EST

Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.