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

[Lai Jiangshan]

On 2022/1/6 00:45, Sean Christopherson wrote:
> On Wed, Jan 05, 2022, Lai Jiangshan wrote:
> > On Wed, Jan 5, 2022 at 5:54 AM Sean Christopherson <seanjc@xxxxxxxxxx> 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.

Oh, I wrote the negative what I want to say again when I try to emphasis
something after I wrote a sentence and modified it several times.

I wanted to mean "EPT not enabled" when vmx.

The VM entry fails when the guest is in very early stage when booting which
might be still in real mode.

VMEXIT: intr_info=00000000 errorcode=0000000 ilen=00000000
reason=80000021 qualification=0000000000000002

IDTVectoring: info=00000000 errorcode=00000000

> 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.

If you can try this patchset, you can just set the default_pae_pdpte to 0 to test
it.

If you can't try this patchset, the mmu->pae_root can be possible to be modified
to test it.

I guess the vmx fails to translate %rip when VMentry in this case.