Re: [3/8] x86/tsc: Store and check TSC ADJUST MSR

From: Thomas Gleixner
Date: Mon Jan 30 2017 - 05:46:26 EST


On Fri, 27 Jan 2017, Henning Schild wrote:
> did you by any chance look into TSC synchronization by adjusting the
> absolute value (MSR_IA32_TSC) as well? As far as i have seen Linux did
> that a long time ago and eventually it was stopped because it caused more
> harm than good.

I was involved in both developing the TSC sync patches and ripping them out

The problem with writing TSC directly is that you really _CANNOT_ reliably
handle run-time differences and SMI/NMI induced deltas. With the adjust MRS
it's a halfways sane thing to do, except for the brokeness of that botch
job vs. the TSC deadline timer.

> The ADJUST MSR offers an easy way to synchronize, still taking care of
> all the special cases resulted in an 8-patch series. Synching without
> that using the absolute value is likely much harder, but that series
> might be a good foundation.

I'm not even thinking about bringing the pure TSC based sync back.

> The big question is whether we can rely on all future CPUs to
> support that MSR. Do "new MSRs" disappear again at some point? If we
> can not rely on the ADJUST MSR, now might be a good time to revisit the
> idea of synching the absolute values.

There is nothing you can ever be sure about, but I doubt that the ADJUST
MSR is going to vanish.

> I remember having read somewhere that this series might get backported
> to longterm kernels, what is the status on that?

No idea.