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

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 12/10/15 10:43, Tian, Kevin wrote:
> > From: Andrew Cooper [mailto:andrew.cooper3@citrix.com]
> > 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).


Haozhong