From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH 13/13] tools/libxl: Add 'vtsc_khz' option to
 set guest TSC rate
Date: Tue, 29 Sep 2015 11:24:36 +0100
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To: Ian Campbell <ian.campbell@citrix.com>, xen-devel@lists.xen.org, Ian
	Jackson <ian.jackson@eu.citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Wei Liu <wei.liu2@citrix.com>, Keir
	Fraser <keir@xen.org>, Jan Beulich <jbeulich@suse.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>, Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>, Jun Nakajima <jun.nakajima@intel.com>, Kevin Tian <kevin.tian@intel.com>
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On 29/09/15 11:13, Haozhong Zhang wrote:
> On Tue, Sep 29, 2015 at 11:04:14AM +0100, Ian Campbell wrote:
>> On Mon, 2015-09-28 at 15:13 +0800, Haozhong Zhang wrote:
>>> This patch adds an option 'vtsc_khz' to allow users to set vcpu's TSC
>>> rate in KHz. In the case that tsc_mode = 'default', the default value of
>>> 'vtsc_khz' option is the host TSC rate which is used when 'vtsc_khz'
>>> option is set to 0 or does not appear in the configuration. In all other
>>> cases of tsc_mode, 'vtsc_khz' option is just ignored.
>>>
>>> Another purpose of adding this option is to keep vcpu's TSC rate across
>>> guest reboot. In existing code, a new domain is created from the
>>> configuration of the previous domain which was just rebooted. vcpu's TSC
>>> rate is not stored in the configuration and the host TSC rate is the
>>> used as vcpu's TSC rate. This works fine unless the previous domain was
>>> migrated from another host machine with a different host TSC rate than
>>> the current one.
>> I understand why this is necessary over a migration, but why is it
>> important to be able to retain the TSC rate across a reboot? What is the
>> usecase there?
>>
> No usecase so far. Is 'making a virtual machine more like a physical
> machine' reasonable here? (I suppose a physical machine retains TSC
> rate across reboot)

There are situations such as altering firmware settings which can cause
the TSC rate to differ across reboot.  I don't see any reason to try and
maintain it across VM reboots.

~Andrew