[RFC PATCH 2/8] KVM: x86/mmu: Use bits 2:0 to check for present SPTEs

[Sean Christopherson <sean.j.christopherson@xxxxxxxxx>]

Use what are effectively EPT's RWX bits to detect present SPTEs instead
of simply looking for a non-zero value.  This will allow using a
non-zero initial value for SPTEs as well as using not-present SPTEs to
track metadata for zapped private SPTEs.

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c         | 9 +++++++--
 arch/x86/kvm/mmu/paging_tmpl.h | 3 ++-
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d737042fea55e..82f69a7456004 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -625,9 +625,14 @@ static int is_nx(struct kvm_vcpu *vcpu)
 	return vcpu->arch.efer & EFER_NX;
 }
 
-static int is_shadow_present_pte(u64 pte)
+static inline bool __is_shadow_present_pte(u64 pte)
 {
-	return (pte != 0) && !is_mmio_spte(pte);
+	return !!(pte & 0x7);
+}
+
+static bool is_shadow_present_pte(u64 pte)
+{
+	return __is_shadow_present_pte(pte) && !is_mmio_spte(pte);
 }
 
 static int is_large_pte(u64 pte)
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 0172a949f6a75..57813e92ea8e0 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -1024,7 +1024,8 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
 		gpa_t pte_gpa;
 		gfn_t gfn;
 
-		if (!sp->spt[i])
+		if (!__is_shadow_present_pte(sp->spt[i]) &&
+		    !is_mmio_spte(sp->spt[i]))
 			continue;
 
 		pte_gpa = first_pte_gpa + i * sizeof(pt_element_t);
-- 
2.28.0