Re: QUESTION: mmu.gva_to_gpa versus nested_mmu.gva_to_gpa

[Paolo Bonzini <pbonzini@redhat.com>]

On 28/12/2015 23:23, David Matlack wrote:
> I'm wondering if this comment in mmu.c:init_kvm_nested_mmu is correct (at
> least in the context of Nested EPT):
> 
> 4055         /*
> 4056          * Note that arch.mmu.gva_to_gpa translates l2_gva to l1_gpa. The
> 4057          * translation of l2_gpa to l1_gpa addresses is done using the
> 4058          * arch.nested_mmu.gva_to_gpa function. Basically the gva_to_gpa
> 4059          * functions between mmu and nested_mmu are swapped.
> 4060          */
> 
> nested_mmu.get_cr3 gets set to get_cr3, which I believe will return L2's cr3.
> In vmx.c:nested_ept_init_mmu_context, mmu.get_cr3 is set to
> nested_ept_get_cr3, which should be the root of EPT12. Given these get_cr3
> functions, shouldn't nested_mmu.gva_to_gpa translate l2_gva->l2_gpa and
> mmu.gva_to_gpa translate l2_gpa->l1_gpa?

Yes, it's correct.  It can be trivially seen by looking at
kvm_init_shadow_ept_mmu's usage of &vcpu->arch.mmu.  This is obviously a
l2_gpa to l1_gpa translation.

Whether the roles are swapped, depends on whether you think of
"nested_mmu" as "nested guest" or "nested virtualization"  nested_mmu is
the MMU for the nested guest, mmu is the MMU for the L1 guest and it's
the one that takes care of nested virtualization.

Paolo