Re: [Qemu-devel] [PATCH v2 0/3] target-i386: save/restore vcpu's TSC rate during migration

[Eduardo Habkost]

On Fri, Oct 23, 2015 at 08:35:20AM -0200, Marcelo Tosatti wrote:
> On Thu, Oct 22, 2015 at 04:45:21PM -0200, Eduardo Habkost wrote:
> > On Tue, Oct 20, 2015 at 03:22:51PM +0800, Haozhong Zhang wrote:
> > > This patchset enables QEMU to save/restore vcpu's TSC rate during the
> > > migration. When cooperating with KVM which supports TSC scaling, guest
> > > programs can observe a consistent guest TSC rate even though they are
> > > migrated among machines with different host TSC rates.
> > > 
> > > A pair of cpu options 'save-tsc-freq' and 'load-tsc-freq' are added to
> > > control the migration of vcpu's TSC rate.
> > 
> > The requirements and goals aren't clear to me. I see two possible use
> > cases, here:
> > 
> > 1) Best effort to keep TSC frequency constant if possible (but not
> >    aborting migration if not possible). This would be an interesting
> >    default, but a bit unpredictable.
> > 2) Strictly ensuring TSC frequency stays constant on migration (and
> >    aborting migration if not possible). This would be an useful feature,
> >    but can't be enabled by default unless both hosts have the same TSC
> >    frequency or support TSC scaling.
> 
> Only destination needs to support TSC scaling, to match the frequency
> of the incoming host.

True.

> 
> The KVM code for this feature has submitted or integrated? 
> 
> > Which one(s) you are trying to implement?
> > 
> > In other words, what is the right behavior when KVM_SET_TSC_KHZ fails or
> > KVM_CAP_TSC_CONTROL is not available? We can't answer that question if
> > the requirements and goals are not clear.
> > 
> > Once we know what exactly is the goal, we could enable the new mode with
> > a single option, instead of raw options to control migration stream
> > loading/saving.
> 
> Windows and Linux guests have paravirt clocks and/or options to
> disable direct TSC usage for timekeeping purposes. So disabling
> migration seems overkill.

I assume that users who set TSC frequency explicitly in the VM config
care about it (otherwise they wouldn't be setting it explicitly) and
don't want it to change after migration.

-- 
Eduardo