Messages in this thread Patch in this message | | | Date | Tue, 24 Apr 2018 10:06:09 +0200 | From | Peter Zijlstra <> | Subject | Re: Re: [PATCH] x86: tsc: fix L2 guest always in tsc_early clocksource |
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On Tue, Apr 24, 2018 at 09:56:52AM +0800, peng.hao2@zte.com.cn wrote: > >On Tue, Apr 24, 2018 at 09:08:54AM +0800, Peng Hao wrote: > >> In L2 guest tsc_read_refs always return ULLONG_MAX, and that will > >> call tsc_refine_calibration_work periodly. So L2 guest will read > >> acpi timer port 0x608 periodly. > >> The patch will let it out of "if(tsc_start == -1){}" infinite loop. > > >Help me out a little. What's an L2 guest?
> kvm nested virtual machine.
So why not say nested guest? I though someone did a new hypervisor based on L4 or so and called it L2 for funnies.
> >So if the whole TSC sync has no way of ever working; what is your patch > >doing? How is the actual TSC calibrated?
> It exists a period loop in tsc_refine_calibration_work: tsc_start = tsc_read_refs() = -1,so it call schedule_delayed_work(&tsc_irqwork,HZ), > then tsc_start = tsc_read_refs() = -1 .... > during the process it accesses 0x608 periodly and it is unnecessary and just influencees performance for L2 guest. > The patch let it out of the period loop and changes from early-tsc to tsc clocksource.So it will not access ioport 0x608 > periodly.
Can you please teach your mailer to wrap at 78 chars?
> The kvmclock give a interface kvm_get_tsc_khz to get the tsc_khz.
But why are you getting to tsc_refine_calibration_work() in the first place? If kvm_get_tsc_khz() returns a known good value, it should set X86_FEATURE_TSC_KNOWN_FREQ such that we don't attempt to 'refine' things.
Something like the below perhaps?
--- arch/x86/kernel/kvmclock.c | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c index 8b26c9e01cc4..918948eaa232 100644 --- a/arch/x86/kernel/kvmclock.c +++ b/arch/x86/kernel/kvmclock.c @@ -138,6 +138,13 @@ static unsigned long kvm_get_tsc_khz(void) src = &hv_clock[cpu].pvti; tsc_khz = pvclock_tsc_khz(src); put_cpu(); + + /* + * TSC frequency is reported by the host; calibration against (virtual) + * HPET/PM-timer in a guest is dodgy and will not be accurate. + */ + setup_force_cpu_cap(X86_FEATURE_TSC_KNOWN_FREQ); + return tsc_khz; }
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