Re: [Qemu-devel] [PATCH v2 3/3] target-i386: load the migrated vcpu's TSC rate

[Eduardo Habkost]

On Fri, Oct 23, 2015 at 11:14:48AM +0800, Haozhong Zhang wrote:
> On Thu, Oct 22, 2015 at 04:11:37PM -0200, Eduardo Habkost wrote:
> > On Tue, Oct 20, 2015 at 03:22:54PM +0800, Haozhong Zhang wrote:
> > > Set vcpu's TSC rate to the migrated value (if any). If KVM supports TSC
> > > scaling, guest programs will observe TSC increasing in the migrated rate
> > > other than the host TSC rate.
> > > 
> > > The loading is controlled by a new cpu option 'load-tsc-freq'. If it is
> > > present, then the loading will be enabled and the migrated vcpu's TSC
> > > rate will override the value specified by the cpu option
> > > 'tsc-freq'. Otherwise, the loading will be disabled.
> > 
> > Why do we need an option? Why can't we enable loading unconditionally?
> >
> 
> If TSC scaling is not supported by KVM and CPU, unconditionally
> enabling this loading will not take effect which would be different
> from users' expectation. 'load-tsc-freq' is introduced to allow users
> to enable the loading of migrated TSC frequency if they do know the
> underlying KVM and CPU have TSC scaling support.
> 

I don't get your argument about user expectations. We can't read the
user's mind, but let's enumerate all possible scenarios:

* Host has TSC scaling, user expect TSC frequency to be set:
  * We set it. The user is happy.
* Host has TSC scaling, user doesn't expect TSC frequency to be
  set:
  * We still set it. VM behaves better, guest doesn't see changing TSC
    frequency. User didn't expect it but won't be unhappy.
* No TSC scaling, user expect TSC frequency to be set:
  * We won't set it, user will be unhappy. But I believe we all agree
    we shouldn't make QEMU abort migration by default on all hosts that
    don't support TSC scaling.
* No TSC scaling, user doesn't expect TSC frequency to be set:
  * We don't set it. User is happy.

Could you clarify on which items you disagree above, exactly?

-- 
Eduardo