Re: [PATCH 02/22] kvm: mmu: Introduce tdp_iter

[Sean Christopherson]

On Thu, Oct 01, 2020 at 01:34:53AM +0200, Paolo Bonzini wrote:
> On 01/10/20 01:20, Eric van Tassell wrote:
> >>
> >> +int is_shadow_present_pte(u64 pte)
> >>   {
> >>       return (pte != 0) && !is_mmio_spte(pte);
> > From <Figure 28-1: Formats of EPTP and EPT Paging-Structure Entries" of
> > the manual I don't have at my fingertips right now, I believe you should
> > only check the low 3 bits(mask = 0x7). Since the upper bits are ignored,
> > might that not mean they're not guaranteed to be 0?
> 
> No, this a property of the KVM MMU (and how it builds its PTEs) rather
> than the hardware present check.

Ya, I found out the hard way that "present" in is_shadow_present_pte() really
means "valid", or "may be present".  The most notable case is EPT without A/D
bits (I think this is the only case where a valid SPTE can be fully not-present
in hardware).  Accessed tracking will clear all RWX bits to make the EPT entry
not-present, but from KVM's perspective it's treated as valid/present because
it can be made present in hardware without taking the MMU lock.