Re: [PATCH 1/12] KVM: X86: Fix when shadow_root_level=5 && guest root_level<4

[Sean Christopherson]

On Thu, Dec 09, 2021, Sean Christopherson wrote:
> On Wed, Nov 24, 2021, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > 
> > If the is an L1 with nNPT in 32bit, the shadow walk starts with
> > pae_root.
> > 
> > Fixes: a717a780fc4e ("KVM: x86/mmu: Support shadowing NPT when 5-level paging is enabled in host)
> 
> Have you actually run with 5-level nNPT?  I don't have access to hardware, at least
> not that I know of :-)
> 
> I'm staring at kvm_mmu_sync_roots() and don't see how it can possibly work for
> 5-level nNPT with a 4-level NPT guest.

Oh, and fast_pgd_switch() will also break kvm_mmu_sync_prev_roots() / is_unsync_root()
by putting a root into the prev_roots array that doesn't have a shadow page associated
with the root.