Re: [Xen-devel] Xen-4.4-rc2 - Some Nested Virt testing

[Tim Deegan <tim@xxxxxxx>]

B0;278;0cAt 08:29 +0000 on 26 Jan (1390721344), Zhang, Yang Z wrote:
> George Dunlap wrote on 2014-01-25:
> > On Tue, Jan 21, 2014 at 6:12 PM, Andrew Cooper
> > <andrew.cooper3@xxxxxxxxxx> wrote:
> >> Hello,
> >> 
> >> I have been giving nested virt a try, and have my first bug to report.
> >> This is still ongoing, and is by no means complete yet.
> >> 
> >> Setup:
> >> Each reference to XenServer is a trunk XenServer based on 4.4-rc2
> >> 
> >> Single Intel Haswell SDP (Grantley platform):
> >> Native hypervisor: XenServer
> >> 
> >> Two L1 guests:
> >>   XenServer (running with EPT)
> >>   XenServer (running with shadow)
> >> 
> >> When attempting to create an L2 EPT HVM domain under an L1 shadow
> >> domain, the L1 shadow domain is killed with:
> > 
> > Is EPT-on-shadow actually meant to work?  I wouldn't be surprised if
> > the L2 HAP stuff assumed that L1 was HAP as well.
> > 
> > In which case, if an L1 guest is started in shadow mode, then EPT
> > should not be advertised.
> 
> AFAK, EPT-on-shadow is not supported. Shadow-on-shadow is buggy
> (Actually, I never tried it successfully from the first day I start
> working on nested stuff).

Fair enough.  That needs to be documented, and those modes (which I
guess means nested-on-shadow in general) need to be disabled in the
hypervisor, with a sensible error message.

> Shadow-on-EPT and EPT-on-EPT are working
> in my box. But I recommended using EPT on EPT if possible. Because
> it is really a pain to run L2 guest on shadow on shadow mode due to
> the poor performance.

Yeah, I think it's generally accepted that having shadow pagetables
anywhere in that stack is going to hurt.  Sadly, there's no way for
the L0 admin to stop the L1 hypervisor from using shadow pagetables,
so shadow-on-EPT ought to at least work correctly, even if performance
sucks.

Tim.

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