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

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 13, 2022, Paolo Bonzini wrote:
> On 4/12/22 23:31, Sean Christopherson wrote:
> > > +		unsigned glevel:4;
> > 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;
> > 
> > Where the field is ignored if direct=1, '0' for non-passthrough, and 1-3 to handle
> > shadow_root_level - guest_root_level.  Basically the same idea as Paolo's smushing
> > of direct+passthrough into mapping_level, just dressed up differently.
> 
> 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?

> 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.

> * If we go with yours, I would call the field "passthrough_levels".

Hmm, it's not a raw level though.