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

[Lai Jiangshan]

On 2022/1/7 03:41, Sean Christopherson wrote:
> On Thu, Jan 06, 2022, Lai Jiangshan wrote:
> > 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.
>
> Heh, that makes a lot more sense.
>
> > 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
>
> Yep, that's the signature for an illegal PDPTE at VM-Enter.  But as noted above,
> a not-present PDPTE is perfectly legal, VM-Enter should failed if and only if a
> PDPTE is present and has reserved bits set.
>
> > 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.
>
> I can't reproduce the failure with this on top of your series + kvm/queue (commit
> cc0e35f9c2d4 ("KVM: SVM: Nullify vcpu_(un)blocking() hooks if AVIC is disabled")).


I can't reproduce the failure with this code base either.  And I can't reproduce
the failure when I switch to the code base when I developed it.

After reviewing all the logs I saved that time, I think it was fixed after
make_pae_pdpte().  I should have added make_pae_pdpte() first before added
default_pae_pdpte.  (The code was still mess and the guest can't fully
function even when make_pae_pdpte() was added that time)

Removing default_pae_pdpte will simplify the code. Thank you.

Thanks
Lai.