KVM: x86/mmu: Add helper to convert SPTE value to its shadow page

Add a helper to convert a SPTE to its shadow page to deduplicate a
variety of flows and hopefully avoid future bugs, e.g. if KVM attempts to
get the shadow page for a SPTE without dropping high bits.

Opportunistically add a comment in mmu_free_root_page() documenting why
it treats the root HPA as a SPTE.

No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221019165618.927057-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -1819,7 +1819,7 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp,
 			continue;
 		}
 
-		child = to_shadow_page(ent & SPTE_BASE_ADDR_MASK);
+		child = spte_to_child_sp(ent);
 
 		if (child->unsync_children) {
 			if (mmu_pages_add(pvec, child, i))
@@ -2378,7 +2378,7 @@ static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep,
 		 * so we should update the spte at this point to get
 		 * a new sp with the correct access.
 		 */
-		child = to_shadow_page(*sptep & SPTE_BASE_ADDR_MASK);
+		child = spte_to_child_sp(*sptep);
 		if (child->role.access == direct_access)
 			return;
 
@@ -2399,7 +2399,7 @@ static int mmu_page_zap_pte(struct kvm *kvm, struct kvm_mmu_page *sp,
 		if (is_last_spte(pte, sp->role.level)) {
 			drop_spte(kvm, spte);
 		} else {
-			child = to_shadow_page(pte & SPTE_BASE_ADDR_MASK);
+			child = spte_to_child_sp(pte);
 			drop_parent_pte(child, spte);
 
 			/*
@@ -2838,7 +2838,7 @@ static int mmu_set_spte(struct kvm_vcpu *vcpu, struct kvm_memory_slot *slot,
 			struct kvm_mmu_page *child;
 			u64 pte = *sptep;
 
-			child = to_shadow_page(pte & SPTE_BASE_ADDR_MASK);
+			child = spte_to_child_sp(pte);
 			drop_parent_pte(child, sptep);
 			flush = true;
 		} else if (pfn != spte_to_pfn(*sptep)) {
@@ -3455,7 +3455,11 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
 	if (!VALID_PAGE(*root_hpa))
 		return;
 
-	sp = to_shadow_page(*root_hpa & SPTE_BASE_ADDR_MASK);
+	/*
+	 * The "root" may be a special root, e.g. a PAE entry, treat it as a
+	 * SPTE to ensure any non-PA bits are dropped.
+	 */
+	sp = spte_to_child_sp(*root_hpa);
 	if (WARN_ON(!sp))
 		return;
 
@@ -3940,8 +3944,7 @@ void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)
 		hpa_t root = vcpu->arch.mmu->pae_root[i];
 
 		if (IS_VALID_PAE_ROOT(root)) {
-			root &= SPTE_BASE_ADDR_MASK;
-			sp = to_shadow_page(root);
+			sp = spte_to_child_sp(root);
 			mmu_sync_children(vcpu, sp, true);
 		}
 	}

arch/x86/kvm/mmu/mmu_internal.h

@@ -133,18 +133,6 @@ struct kvm_mmu_page {
 
 extern struct kmem_cache *mmu_page_header_cache;
 
-static inline struct kvm_mmu_page *to_shadow_page(hpa_t shadow_page)
-{
-	struct page *page = pfn_to_page(shadow_page >> PAGE_SHIFT);
-
-	return (struct kvm_mmu_page *)page_private(page);
-}
-
-static inline struct kvm_mmu_page *sptep_to_sp(u64 *sptep)
-{
-	return to_shadow_page(__pa(sptep));
-}
-
 static inline int kvm_mmu_role_as_id(union kvm_mmu_page_role role)
 {
 	return role.smm ? 1 : 0;

arch/x86/kvm/mmu/spte.h

@@ -219,6 +219,23 @@ static inline int spte_index(u64 *sptep)
  */
 extern u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;
 
+static inline struct kvm_mmu_page *to_shadow_page(hpa_t shadow_page)
+{
+	struct page *page = pfn_to_page((shadow_page) >> PAGE_SHIFT);
+
+	return (struct kvm_mmu_page *)page_private(page);
+}
+
+static inline struct kvm_mmu_page *spte_to_child_sp(u64 spte)
+{
+	return to_shadow_page(spte & SPTE_BASE_ADDR_MASK);
+}
+
+static inline struct kvm_mmu_page *sptep_to_sp(u64 *sptep)
+{
+	return to_shadow_page(__pa(sptep));
+}
+
 static inline bool is_mmio_spte(u64 spte)
 {
 	return (spte & shadow_mmio_mask) == shadow_mmio_value &&

arch/x86/kvm/mmu/tdp_mmu.h

@@ -5,6 +5,8 @@
 
 #include <linux/kvm_host.h>
 
+#include "spte.h"
+
 hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
 
 __must_check static inline bool kvm_tdp_mmu_get_root(struct kvm_mmu_page *root)