Messages in this thread | | | Date | Thu, 25 Aug 2016 09:43:51 -0700 | From | Bin Gao <> | Subject | Re: [PATCH v2] x86/tsc: Set X86_FEATURE_TSC_RELIABLE to skip refined calibration |
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On Wed, Aug 24, 2016 at 10:51:20AM +0200, Thomas Gleixner wrote: > On Tue, 16 Aug 2016, Bin Gao wrote: > > On some newer Intel x86 processors/SoCs the TSC frequency can be directly > > calculated by factors read from specific MSR registers or from a cpuid > > leaf (0x15). TSC frequency calculated by native msr/cpuid is absolutely > > accurate so we should always skip calibrating TSC aginst another clock, > > e.g. PIT, HPET, etc. So we want to skip the refined calibration by setting > > the X86_FEATURE_TSC_RELIABLE flag. Existing code setting the flag by > > set_cpu_cap() doesn't work as the flag is cleared later in identify_cpu(). > > A cpu caps flag is not cleared only if it's set by setup_force_cpu_cap(). > > This patch converted set_cpu_cap() to setup_force_cpu_cap() to ensure > > refined calibration is skipped. > > > > We had a test on Intel CherryTrail platform: the 24 hours time drift is > > 3.6 seconds if refined calibration was not skipped while the drift is less > > than 0.6 second when refined calibration was skipped. > > > > Correctly setting the X86_FEATURE_TSC_RELIABLE flag also guarantees TSC is > > not monitored by timekeeping watchdog because on most of these system TSC > > is the only reliable clocksource. HPET, for instance, works but may not > > be reliable. So kernel may report a physically reliable TSC is not reliable > > just because a physically not reliable HPET is acting as timekeeping > > watchdog. > > What about non SoC systems where the MSR is available, but we still see that > cross socket TSC wreckage? This change will prevent the watchdog from > detecting that.
MSR is only available on Intel Atom SoCs. There is no such a multi-socket system.
> > Thanks, > > tglx
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