From mboxrd@z Thu Jan  1 00:00:00 1970
From: Olaf Hering <olaf@aepfle.de>
Subject: Re: [PATCH] xen: preserve native TSC speed during
 migration between identical hosts
Date: Wed, 24 May 2017 17:44:56 +0200
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To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Wei Liu <wei.liu2@citrix.com>, xen-devel@lists.xen.org, Ian Jackson <ian.jackson@eu.citrix.com>, Jan Beulich <jbeulich@suse.com>, Andrew Cooper <andrew.cooper3@citrix.com>
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Am Wed, 24 May 2017 11:33:15 -0400
schrieb Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>:

> But it does not help customers to figure out if this is OK for them?
> As in, how can customers be assured that 1% jitter is OK for their
> kernel? That time won't go backwards?

Well, that would be some new documentation.
I think the tsc_mode=3Dnative part already lacks that info.

In my testing time does not go backwards. The actual value of tsc
is different anyway on each host. Querying an ntp server shows no
drift, the offset remains within the +- 0.5 sec range.

> Is there some form of tests that they can run to verify and test
> that this is safe? Or perhaps this is something that is based on the kern=
el
> versions? Like 4.11 are safe, but 3.18 is not?

I'm not aware of any tests to measure the results of such jitter
with tsc_mode=3Dnative.

Olaf

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