KVM: x86/mmu: do not consult levels when freeing roots

Right now, PGD caching requires a complicated dance of first computing
the MMU role and passing it to __kvm_mmu_new_pgd(), and then separately calling
kvm_init_mmu().

Part of this is due to kvm_mmu_free_roots using mmu->root_level and
mmu->shadow_root_level to distinguish whether the page table uses a single
root or 4 PAE roots.  Because kvm_init_mmu() can overwrite mmu->root_level,
kvm_mmu_free_roots() must be called before kvm_init_mmu().

However, even after kvm_init_mmu() there is a way to detect whether the
page table may hold PAE roots, as root.hpa isn't backed by a shadow when
it points at PAE roots.  Using this method results in simpler code, and
is one less obstacle in moving all calls to __kvm_mmu_new_pgd() after the
MMU has been initialized.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -3224,12 +3224,15 @@ void kvm_mmu_free_roots(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
 	struct kvm *kvm = vcpu->kvm;
 	int i;
 	LIST_HEAD(invalid_list);
-	bool free_active_root = roots_to_free & KVM_MMU_ROOT_CURRENT;
+	bool free_active_root;
 
 	BUILD_BUG_ON(KVM_MMU_NUM_PREV_ROOTS >= BITS_PER_LONG);
 
 	/* Before acquiring the MMU lock, see if we need to do any real work. */
-	if (!(free_active_root && VALID_PAGE(mmu->root.hpa))) {
+	free_active_root = (roots_to_free & KVM_MMU_ROOT_CURRENT)
+		&& VALID_PAGE(mmu->root.hpa);
+
+	if (!free_active_root) {
 		for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
 			if ((roots_to_free & KVM_MMU_ROOT_PREVIOUS(i)) &&
 			    VALID_PAGE(mmu->prev_roots[i].hpa))
@@ -3247,8 +3250,7 @@ void kvm_mmu_free_roots(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
 					   &invalid_list);
 
 	if (free_active_root) {
-		if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
-		    (mmu->root_level >= PT64_ROOT_4LEVEL || mmu->direct_map)) {
+		if (to_shadow_page(mmu->root.hpa)) {
 			mmu_free_root_page(kvm, &mmu->root.hpa, &invalid_list);
 		} else if (mmu->pae_root) {
 			for (i = 0; i < 4; ++i) {