From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <ian.campbell@citrix.com>
Subject: Re: [PATCH 13/13] tools/libxl: Add 'vtsc_khz' option to
 set guest TSC rate
Date: Tue, 29 Sep 2015 11:04:14 +0100
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To: Haozhong Zhang <haozhong.zhang@intel.com>, xen-devel@lists.xen.org
Cc: Kevin Tian <kevin.tian@intel.com>, Wei Liu <wei.liu2@citrix.com>, Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Jun Nakajima <jun.nakajima@intel.com>, Andrew Cooper <andrew.cooper3@citrix.com>, Ian Jackson <ian.jackson@eu.citrix.com>, Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>, Jan Beulich <jbeulich@suse.com>, Keir Fraser <keir@xen.org>, Boris Ostrovsky <boris.ostrovsky@oracle.com>
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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?

> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> ---
>  tools/libxl/libxl_types.idl |  1 +
>  tools/libxl/libxl_x86.c     |  4 +++-
>  tools/libxl/xl_cmdimpl.c    | 22 ++++++++++++++++++++++

The documentation should be patched at the same time. At least the xl.cfg
manpage, but I think there is also a specific document about time and the
TSC which should also be updated.

Ian.