Messages in this thread | | | Date | Mon, 30 Jan 2017 13:13:01 +0100 | From | Henning Schild <> | Subject | Re: [3/8] x86/tsc: Store and check TSC ADJUST MSR |
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On Mon, 30 Jan 2017 11:20:25 +0100 Thomas Gleixner <tglx@linutronix.de> wrote:
> Henning, > > 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 again. > > 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.
That sounds very much like i expected. But assuming the MSR has come to stay, the problem should be solved for recent kernels and Intel-CPUs.
The AMD-Manual from 12/16 does not mention that MSR. I do not have access to an AMD machine. But i can only assume that bigger machines also suffer from async TSCs and basically all fall back to HPET.
> > I remember having read somewhere that this series might get > > backported to longterm kernels, what is the status on that? > > No idea. > > Thanks,
Thanks a lot! Henning
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