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

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

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)

> > 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
                                     ~~~~~~~~~~~~~~~~~~~
I think it's doc/misc/tscmode.txt? Will update it as well.

- Haozhong

> TSC which should also be updated.
>
> Ian.

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