Re: [RFC PATCH 5/6] KVM: X86: Alloc pae_root shadow page

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 05, 2022, Lai Jiangshan wrote:
> On Wed, Jan 5, 2022 at 5:54 AM Sean Christopherson <seanjc@google.com> wrote:
> 
> > >
> > > default_pae_pdpte is needed because the cpu expect PAE pdptes are
> > > present when VMenter.
> >
> > That's incorrect.  Neither Intel nor AMD require PDPTEs to be present.  Not present
> > is perfectly ok, present with reserved bits is what's not allowed.
> >
> > Intel SDM:
> >   A VM entry that checks the validity of the PDPTEs uses the same checks that are
> >   used when CR3 is loaded with MOV to CR3 when PAE paging is in use[7].  If MOV to CR3
> >   would cause a general-protection exception due to the PDPTEs that would be loaded
> >   (e.g., because a reserved bit is set), the VM entry fails.
> >
> >   7. This implies that (1) bits 11:9 in each PDPTE are ignored; and (2) if bit 0
> >      (present) is clear in one of the PDPTEs, bits 63:1 of that PDPTE are ignored.
> 
> But in practice, the VM entry fails if the present bit is not set in the
> PDPTE for the linear address being accessed (when EPT enabled at least).  The
> host kvm complains and dumps the vmcs state.

That doesn't make any sense.  If EPT is enabled, KVM should never use a pae_root.
The vmcs.GUEST_PDPTRn fields are in play, but those shouldn't derive from KVM's
shadow page tables.

And I doubt there is a VMX ucode bug at play, as KVM currently uses '0' in its
shadow page tables for not-present PDPTEs.

If you can post/provide the patches that lead to VM-Fail, I'd be happy to help
debug.