Re: [Xen-devel] [PATCH v2 00/14] Add VMX TSC scaling support

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Tuesday, December 08, 2015 1:04 AM
> 
> On 07/12/15 10:16, Haozhong Zhang wrote:
> > On 12/07/15 10:03, Egger, Christoph wrote:
> >> Did you consider nested virtualization?
> >> L1 hypervisor may have a different tsc scaling
> >> and L2 guest again may have a different tsc scale ratio.
> >>
> > Oh, I forgot this. I'll check the nested TSC scaling code (mostly
> > about nested SVM TSC ratio, because this patch series does not expose
> > VMX TSC scaling to L1 guest).
> >
> > BTW, are there any practical usage scenarios of nested TSC scaling? If
> > any, I may also need to consider adding nested virtualization support
> > to VMX TSC scaling.
> 
> I would say that there are genuine uses for nested TSC scaling.  An L1
> hypervisor is going to want to scale for the same reasons as the L0
> hypervisor.
> 
> Having said that, if TSC scaling is correctly hidden from the L1 guests,
> I would advise against conflating the two issues together.  i.e. getting
> nested TSC scaling working is not a prerequisite for accepting this series.
> 

Why is nested TSC scaling even an issue? If L0 hypervisors cross hosts
can always guarantee constant TSC frequency through TSC scaling, L1
hypervisor will never see inconstant TSC frequency so why bother to
expose TSC scaling at all?

Thanks
Kevin

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