KVM: x86/mmu: look for a cached PGD when going from 32-bit to 64-bit

Right now, PGD caching avoids placing a PAE root in the cache by using the
old value of mmu->root_level and mmu->shadow_root_level; it does not look
for a cached PGD if the old root is a PAE one, and then frees it using
kvm_mmu_free_roots.

Change the logic instead to free the uncacheable root early.
This way, __kvm_new_mmu_pgd is able to look up the cache when going from
32-bit to 64-bit (if there is a hit, the invalid root becomes the least
recently used).  An example of this is nested virtualization with shadow
paging, when a 64-bit L1 runs a 32-bit L2.

As a side effect (which is actually the reason why this patch was
written), PGD caching does not use the old value of mmu->root_level
and mmu->shadow_root_level anymore.
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -4085,52 +4085,88 @@ static inline bool is_root_usable(struct kvm_mmu_root_info *root, gpa_t pgd,
 				  union kvm_mmu_page_role role)
 {
 	return (role.direct || pgd == root->pgd) &&
-	       VALID_PAGE(root->hpa) && to_shadow_page(root->hpa) &&
+	       VALID_PAGE(root->hpa) &&
 	       role.word == to_shadow_page(root->hpa)->role.word;
 }
 
 /*
- * Find out if a previously cached root matching the new pgd/role is available.
- * The current root is also inserted into the cache.
- * If a matching root was found, it is assigned to kvm_mmu->root.hpa and true is
- * returned.
- * Otherwise, the LRU root from the cache is assigned to kvm_mmu->root.hpa and
- * false is returned. This root should now be freed by the caller.
+ * Find out if a previously cached root matching the new pgd/role is available,
+ * and insert the current root as the MRU in the cache.
+ * If a matching root is found, it is assigned to kvm_mmu->root and
+ * true is returned.
+ * If no match is found, kvm_mmu->root is left invalid, the LRU root is
+ * evicted to make room for the current root, and false is returned.
  */
-static bool cached_root_available(struct kvm_vcpu *vcpu, gpa_t new_pgd,
-				  union kvm_mmu_page_role new_role)
+static bool cached_root_find_and_keep_current(struct kvm *kvm, struct kvm_mmu *mmu,
+					      gpa_t new_pgd,
+					      union kvm_mmu_page_role new_role)
 {
 	uint i;
-	struct kvm_mmu *mmu = vcpu->arch.mmu;
 
 	if (is_root_usable(&mmu->root, new_pgd, new_role))
 		return true;
 
 	for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++) {
+		/*
+		 * The swaps end up rotating the cache like this:
+		 *   C   0 1 2 3   (on entry to the function)
+		 *   0   C 1 2 3
+		 *   1   C 0 2 3
+		 *   2   C 0 1 3
+		 *   3   C 0 1 2   (on exit from the loop)
+		 */
 		swap(mmu->root, mmu->prev_roots[i]);
-
 		if (is_root_usable(&mmu->root, new_pgd, new_role))
-			break;
+			return true;
 	}
 
-	return i < KVM_MMU_NUM_PREV_ROOTS;
+	kvm_mmu_free_roots(kvm, mmu, KVM_MMU_ROOT_CURRENT);
+	return false;
 }
 
-static bool fast_pgd_switch(struct kvm_vcpu *vcpu, gpa_t new_pgd,
-			    union kvm_mmu_page_role new_role)
+/*
+ * Find out if a previously cached root matching the new pgd/role is available.
+ * On entry, mmu->root is invalid.
+ * If a matching root is found, it is assigned to kvm_mmu->root, the LRU entry
+ * of the cache becomes invalid, and true is returned.
+ * If no match is found, kvm_mmu->root is left invalid and false is returned.
+ */
+static bool cached_root_find_without_current(struct kvm *kvm, struct kvm_mmu *mmu,
+					     gpa_t new_pgd,
+					     union kvm_mmu_page_role new_role)
 {
-	struct kvm_mmu *mmu = vcpu->arch.mmu;
+	uint i;
+
+	for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
+		if (is_root_usable(&mmu->prev_roots[i], new_pgd, new_role))
+			goto hit;
 
+	return false;
+
+hit:
+	swap(mmu->root, mmu->prev_roots[i]);
+	/* Bubble up the remaining roots.  */
+	for (; i < KVM_MMU_NUM_PREV_ROOTS - 1; i++)
+		mmu->prev_roots[i] = mmu->prev_roots[i + 1];
+	mmu->prev_roots[i].hpa = INVALID_PAGE;
+	return true;
+}
+
+static bool fast_pgd_switch(struct kvm *kvm, struct kvm_mmu *mmu,
+			    gpa_t new_pgd, union kvm_mmu_page_role new_role)
+{
 	/*
-	 * For now, limit the fast switch to 64-bit hosts+VMs in order to avoid
+	 * For now, limit the caching to 64-bit hosts+VMs in order to avoid
 	 * having to deal with PDPTEs. We may add support for 32-bit hosts/VMs
 	 * later if necessary.
 	 */
-	if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
-	    mmu->root_level >= PT64_ROOT_4LEVEL)
-		return cached_root_available(vcpu, new_pgd, new_role);
+	if (VALID_PAGE(mmu->root.hpa) && !to_shadow_page(mmu->root.hpa))
+		kvm_mmu_free_roots(kvm, mmu, KVM_MMU_ROOT_CURRENT);
 
-	return false;
+	if (VALID_PAGE(mmu->root.hpa))
+		return cached_root_find_and_keep_current(kvm, mmu, new_pgd, new_role);
+	else
+		return cached_root_find_without_current(kvm, mmu, new_pgd, new_role);
 }
 
 static void __kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd,
@@ -4138,8 +4174,8 @@ static void __kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd,
 {
 	struct kvm_mmu *mmu = vcpu->arch.mmu;
 
-	if (!fast_pgd_switch(vcpu, new_pgd, new_role)) {
-		kvm_mmu_free_roots(vcpu->kvm, mmu, KVM_MMU_ROOT_CURRENT);
+	if (!fast_pgd_switch(vcpu->kvm, mmu, new_pgd, new_role)) {
+		/* kvm_mmu_ensure_valid_pgd will set up a new root.  */
 		return;
 	}