Date: Wed, 27 Jun 2018 08:32:44 -0700 From: John Baldwin <jhb@FreeBSD.org> To: Andriy Gapon <avg@FreeBSD.org>, FreeBSD Current <freebsd-current@FreeBSD.org> Subject: Re: TSC calibration in virtual machines Message-ID: <729d4477-3c68-e3db-2ebc-0bd4deba61cd@FreeBSD.org> In-Reply-To: <8ac353c5-d188-f432-aab1-86f4ca5fd295@FreeBSD.org> References: <8ac353c5-d188-f432-aab1-86f4ca5fd295@FreeBSD.org>
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On 6/27/18 12:14 AM, Andriy Gapon wrote: > > It seems that TSC calibration in virtual machines sometimes can do more harm > than good. Should we default to trusting the information provided by a hypervisor? > > Specifically, I am observing a problem on GCE instances where calibrated TSC > frequency is about 10% lower than advertised frequency. And apparently the > advertised frequency is the right one. > > I found this thread with similar reports and a variety of workarounds from > administratively disabling the calibration to switching to a different timecounter: > https://lists.freebsd.org/pipermail/freebsd-cloud/2017-January/000080.html I suspect you are probably right that we should just "trust" TSC frequencies provided by a hypervisor. We could perhaps choose to whitelist hypervisors known to provide accurate values if we wanted to be cautious. -- John Baldwin
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