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

[haozhong . zhang]

On Fri, Oct 23, 2015 at 12:58:02PM -0200, Eduardo Habkost wrote:
> 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.
> > 
>

Sorry for the confusion, I changed my mind. The semantics of
'load-tsc-freq' is really confusing and we should not need it at all.

Now, what I want to implement is to migrate the TSC frequency as much
as possible. If it could not, QEMU does not abort the migration.

> 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.
Agree.

> * 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.
Agree.

> * 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.
Agree and display warning messages.

> * No TSC scaling, user doesn't expect TSC frequency to be set:
>   * We don't set it. User is happy.
Agree. This is the current QEMU's behavior, so it's still acceptable.

Thanks,
Haozhong

> 
> Could you clarify on which items you disagree above, exactly?
> 
> -- 
> Eduardo
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