Re: [RFC PATCH V3 2/4] KVM: X86: Introduce role.glevel for level expanded pagetable

[Paolo Bonzini]

On 4/13/22 16:42, Sean Christopherson wrote:
> On Wed, Apr 13, 2022, Paolo Bonzini wrote:
> > On 4/12/22 23:31, Sean Christopherson wrote:
> > > We don't need 4 bits for this.  Crossing our fingers that we never had to shadow
> > > a 2-level guest with a 6-level host, we can do:
> > >
> > > 		unsigned passthrough_delta:2;
> > Basically, your passthrough_delta is level - glevel in Jiangshan's patches.
> > You'll need 3 bits anyway when we remove direct later (that would be
> > passthrough_delta == level).
>
> Are we planning on removing direct?

I think so, it's redundant and the code almost always checks 
direct||passthrough (which would be passthrough_delta > 0 with your scheme).

> > Regarding the naming:
> >
> > * If we keep Jiangshan's logic, I don't like the glevel name very much, any
> > of mapping_level, target_level or direct_level would be clearer?
>
> I don't love any of these names, especially glevel, because the field doesn't
> strictly track the guest/mapping/target/direct level.  That could obviously be
> remedied by making it valid at all times, but then the role would truly need 3
> bits (on top of direct) to track 5-level guest paging.

Yes, it would need 3 bits but direct can be removed.

> > * If we go with yours, I would call the field "passthrough_levels".
>
> Hmm, it's not a raw level though.

Hence the plural. :)

Paolo