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

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Message-ID: <20150821143415.GK3161@worktop.event.rightround.com>
Date:	Fri, 21 Aug 2015 16:34:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	mingo@...nel.org, haiyangz@...rosoft.com, tglx@...utronix.de,
	hpa@...or.com, vkuznets@...hat.com, 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:     http://git.kernel.org/tip/88c9281a9fba67636ab26c1fd6afbc78a632374f
> 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?
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