Re: [Xen-devel] [PATCH 13/13] tools/libxl: Add 'vtsc_khz' option to set guest TSC rate

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

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@xxxxxxxxx>
> ---
>  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.

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