linux-kernel - RE: [tip:x86/platform] x86/hyperv: Mark the Hyper-V TSC as unstable

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Message-ID: <BY2PR0301MB165478811EC76AE22BC84316A0650@BY2PR0301MB1654.namprd03.prod.outlook.com>
Date:	Fri, 21 Aug 2015 15:35:55 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"vkuznets@...hat.com" <vkuznets@...hat.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: RE: [tip:x86/platform] x86/hyperv: Mark the Hyper-V TSC as unstable



> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@...radead.org]
> Sent: Friday, August 21, 2015 7:34 AM
> To: mingo@...nel.org; Haiyang Zhang <haiyangz@...rosoft.com>;
> tglx@...utronix.de; hpa@...or.com; vkuznets@...hat.com; KY Srinivasan
> <kys@...rosoft.com>; torvalds@...ux-foundation.org; linux-
> kernel@...r.kernel.org
> Cc: linux-tip-commits@...r.kernel.org
> Subject: Re: [tip:x86/platform] x86/hyperv: Mark the Hyper-V TSC as
> unstable
> 
> On Fri, Aug 21, 2015 at 12:45:50AM -0700, tip-bot for Vitaly Kuznetsov wrote:
> > Commit-ID:  88c9281a9fba67636ab26c1fd6afbc78a632374f
> > Gitweb:
> https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fgit.kern
> el.org%2ftip%2f88c9281a9fba67636ab26c1fd6afbc78a632374f&data=01%7c01
> %7ckys%40microsoft.com%7cca840f9ebba34958dc9a08d2aa35a6c4%7c72f988
> bf86f141af91ab2d7cd011db47%7c1&sdata=hzE233wXaLAYShWPE18SQJQvQe
> JguAdB6oj%2bF%2blx7Ok%3d
> > Author:     Vitaly Kuznetsov <vkuznets@...hat.com>
> > AuthorDate: Wed, 19 Aug 2015 09:54:24 -0700
> > Committer:  Ingo Molnar <mingo@...nel.org>
> > CommitDate: Fri, 21 Aug 2015 08:44:38 +0200
> >
> > x86/hyperv: Mark the Hyper-V TSC as unstable
> >
> > The Hyper-V top-level functional specification states, that
> > "algorithms should be resilient to sudden jumps forward or
> > backward in the TSC value", this means that we should consider
> > TSC as unstable. In some cases tsc tests are able to detect the
> > instability, it was detected in 543 out of 646 boots in my
> > testing:
> >
> >  Measured 6277 cycles TSC warp between CPUs, turning off TSC clock.
> >  tsc: Marking TSC unstable due to check_tsc_sync_source failed
> >
> > This is, however, just a heuristic. On Hyper-V platform there
> > are two good clocksources: MSR-based hyperv_clocksource and
> > recently introduced TSC page.
> 
> *groan*.. and where are the paravirt ops like pv_time_ops for hyperv to
> fix up this mess?

We register external clocksource and clockevents devices based on a hypervisor based time source and
timers to deal with this issue.

Regards,

K. Y
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