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

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

> From: Zhang, Haozhong
> Sent: Thursday, December 10, 2015 7:13 PM
> 
> On 12/10/15 10:43, Tian, Kevin wrote:
> > > 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?
> >
> 
> If exposing TSC scaling to L1, then L0 hypervisor will need to adapt
> the TSC scaling ratio set by L1 hypervisor, which has not been done by
> this patch series.
> 
> Consider an example that the host TSC frequency is freq_0 and
> 1. L0 Xen sets TSC scaling ratio to a non-identical one ratio_0.
> 
> 2. Then L1 hypervisor will observe a "host" TSC frequency (freq_0 * ratio_0).
> 
> 3. If L1 hypervisor sets a TSC scaling ratio ratio_1, it intends to
>    provide a guest TSC frequency (freq_0 * ratio_0 * ratio_1).
> 
>    However, if ratio_1 is directly written into nested VMCS that is
>    later applied to the host CPU, then the L2 guest will get an
>    incorrect guest TSC frequency (freq_0 * ratio_1).
> 
> 

My point is why we need expose TSC scaling to L1, if L0 has ensures
constant TSC w/ TSC scaling ratio...

Thanks
Kevin

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