Re: [PATCH 04/16] KVM: x86/mmu: Add address conversion functions for TDX shared bit of GPA

[Huang, Kai]

> > E.g,  why we cannot do:
> >
> >          static bool kvm_use_private_root(struct kvm *kvm)
> >          {
> >                  return kvm->arch.vm_type == VM_TYPE_TDX;
> >          }
> >
> > Or,
> >          static bool kvm_use_private_root(struct kvm *kvm)
> >          {
> >                  return kvm->arch.use_private_root;
> >          }
> >
> > Or, assuming we would love to keep the kvm_gfn_shared_mask():
> >
> >          static bool kvm_use_private_root(struct kvm *kvm)
> >          {
> >                  return !!kvm_gfn_shared_mask(kvm);
> >          }
> >
> > And then:
> >
> > In fault handler:
> >
> >          if (fault->is_private && kvm_use_private_root(kvm))
> >                  // use private root
> >          else
> >                  // use shared/normal root
> >
> > When you zap:
> >
> >          bool private_gpa = kvm_mem_is_private(kvm, gfn);
> >          
> >          if (private_gpa && kvm_use_private_root(kvm))
> >                  // zap private root
> >          else
> >                  // zap shared/normal root.
>
> I think you are trying to say not to abuse kvm_gfn_shared_mask() as is currently
> done in this logic. But we already agreed on this. So not sure.

To be clear:  We agreed on this in general, but not on this 
kvm_on_private_root().

It's obvious that you still want to "use kvm_gfn_shared_mask() to 
determine whether a GPA is private" for this helper but I don't like it. 
 In fact I don't see why we even need this helper.

I think I am just too obsessed on avoiding using kvm_gfn_shared_mask() 
so I'll stop commenting/replying on this.

[...]

> I don't think we can get rid of the shared mask. Even if we relied on
> kvm_mem_is_private() to determine if a GPA is private or shared, at absolute
> minimum we need to add the shared bit when we are zapping a GFN or mapping it.

No we cannot, but we can avoid using it here.

> Let's table the discussion until we have some code to look again.

100% agreed.