Re: [PATCH v2 1/3] KVM: x86: Allow CPU to force vendor-specific TDP level

Yu Zhang
2021-08-10T11:00:42Z

On Tue, Aug 10, 2021 at 11:25:27AM +0200, Paolo Bonzini wrote:
> On 10/08/21 09:40, Yu Zhang wrote:
> > About "host can't easily mirror L1's desired paging mode", could you please elaborate?
> > Thanks!
>
> Shadow pgae tables in KVM will always have 3 levels on 32-bit machines and
> 4/5 levels on 64-bit machines. L1 instead might have any number of levels
> from 2 to 5 (though of course not more than the host has).

Thanks Paolo.

I guess it's because, unlike EPT which are with either 4 or 5 levels, NPT's
level can range from 2 to 5, depending on the host paging mode...

>
> Therefore, when shadowing 32-bit NPT page tables, KVM has to add extra fixed
> levels on top of those that it's shadowing. See mmu_alloc_direct_roots for
> the code.

So when shadowing NPTs(can be 2/3 levels, depending on the paging mode in L1),
and if L0 Linux is running in 4/5 level mode, extra levels of paging structures
is needed in the shadow NPT.

But shadow EPT does not have such annoyance. Is above understanding correct?

B.R.
Yu