linux-kernel - Re: [PATCH] x86, TSC: Add a software TSC offset

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Date:	Mon, 21 Jul 2014 12:34:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] x86, TSC: Add a software TSC offset

On 07/19/2014 06:06 AM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
> 
> There are machines which do have stable and always-running TSCs but the
> last get started at different points in time by the platform, causing
> the TSCs to have a small constant diff.
> 
> It has been tried a couple of times to resync those during that
> sync check but the procedure is error prone and flaky, and not 100%
> successful.
> 
> So, instead of doing that, let's not touch the TSCs at all but save a
> per-CPU TSC offset which we add to the TSC value we've read from the
> Time-Stamp Counter. The hope is thus to still salvage the TSC on those
> machines.
> 
> For that to work, we need to populate the TSC AUX MSR with the core ID
> prior to doing the TSC sync check so that RDTSCP can give us the correct
> core number and we can add the offset atomically. And yes, we need a
> X86_FEATURE_RDTSCP CPU for the whole deal to work. Older ones simply
> lose.

I'm sure I'm missing something, but where is tsc_offset used outside the
tsc startup code?  Is it somehow getting programmed into the TSC config
registers?

--Andy
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