KVM: VMX: fix tlb flush with invalid root

Commit 341d9b535b6c simplify reload logic while entry guest mode, it
can avoid unnecessary sync-root if KVM_REQ_MMU_RELOAD and
KVM_REQ_MMU_SYNC both set.

But, it cause a issue that when we handle 'KVM_REQ_TLB_FLUSH', the
root is invalid, it is triggered during my test:

Kernel BUG at ffffffffa00212b8 [verbose debug info unavailable]
......

Fixed by directly return if the root is not ready.
Signed-off-by: NXiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -40,6 +40,8 @@
 				  0xFFFFFF0000000000ULL)
 
 #define INVALID_PAGE (~(hpa_t)0)
+#define VALID_PAGE(x) ((x) != INVALID_PAGE)
+
 #define UNMAPPED_GVA (~(gpa_t)0)
 
 /* KVM Hugepage definitions for x86 */

arch/x86/kvm/mmu.c

@@ -92,8 +92,6 @@ module_param(oos_shadow, bool, 0644);
 #define PT_FIRST_AVAIL_BITS_SHIFT 9
 #define PT64_SECOND_AVAIL_BITS_SHIFT 52
 
-#define VALID_PAGE(x) ((x) != INVALID_PAGE)
-
 #define PT64_LEVEL_BITS 9
 
 #define PT64_LEVEL_SHIFT(level) \

arch/x86/kvm/vmx.c

@@ -1828,8 +1828,11 @@ static void exit_lmode(struct kvm_vcpu *vcpu)
 static void vmx_flush_tlb(struct kvm_vcpu *vcpu)
 {
 	vpid_sync_context(to_vmx(vcpu));
-	if (enable_ept)
+	if (enable_ept) {
+		if (!VALID_PAGE(vcpu->arch.mmu.root_hpa))
+			return;
 		ept_sync_context(construct_eptp(vcpu->arch.mmu.root_hpa));
+	}
 }
 
 static void vmx_decache_cr0_guest_bits(struct kvm_vcpu *vcpu)