linux-kernel - Re: [v1 0/9] Early boot time stamps for x86

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Date:   Thu, 23 Mar 2017 11:56:17 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Pasha Tatashin <pasha.tatashin@...cle.com>
cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com
Subject: Re: [v1 0/9] Early boot time stamps for x86

On Wed, 22 Mar 2017, Pasha Tatashin wrote:
> Yes, I am certain it is 0 or  near 0 on reset on this machine. Because, I

Emphasis on "this machine'

It's not guaranteed especially not on reboot and not with creative BIOSes
fiddling with the TSC_ADJUST value. 

 - It CANNOT be used to measure BIOS boot time reliably

 - If BIOS wreckaged TSC_ADJUST, then your whole time stamping goes out the
   window once the kernel sanitized it.

See other mail.

Thanks,

	tglx