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

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-09-29 at 11:24 +0100, Andrew Cooper wrote:
> 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.

Right. If it happens to come for free as a side effect of making it work
for migration then fine.

But it seems to me that tsc rate could/should be in the hypervisors save
blob and require no interaction with the toolstack once it is latched when
the domain is built.

Ian.