lkml.org 
[lkml]   [2020]   [Sep]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 02/22] kvm: mmu: Introduce tdp_iter
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.

\
 
 \ /
  Last update: 2020-10-01 02:08    [W:0.066 / U:1.812 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site