kvm: nVMX: don't flush VMCS12 during VMXOFF or VCPU teardown

According to the Intel SDM, software cannot rely on the current VMCS to be
coherent after a VMXOFF or shutdown. So this is a valid way to handle VMCS12
flushes.

24.11.1 Software Use of Virtual-Machine Control Structures
...
  If a logical processor leaves VMX operation, any VMCSs active on
  that logical processor may be corrupted (see below). To prevent
  such corruption of a VMCS that may be used either after a return
  to VMX operation or on another logical processor, software should
  execute VMCLEAR for that VMCS before executing the VMXOFF instruction
  or removing power from the processor (e.g., as part of a transition
  to the S3 and S4 power states).
...

This fixes a "suspicious rcu_dereference_check() usage!" warning during
kvm_vm_release() because nested_release_vmcs12() calls
kvm_vcpu_write_guest_page() without holding kvm->srcu.
Signed-off-by: David Matlack <dmatlack@google.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/vmx.c

@@ -419,7 +419,7 @@ struct nested_vmx {
 	/*
 	 * Cache of the guest's VMCS, existing outside of guest memory.
 	 * Loaded from guest memory during VMPTRLD. Flushed to guest
-	 * memory during VMXOFF, VMCLEAR, VMPTRLD.
+	 * memory during VMCLEAR and VMPTRLD.
 	 */
 	struct vmcs12 *cached_vmcs12;
 	/*
@@ -7174,6 +7174,12 @@ static int nested_vmx_check_permission(struct kvm_vcpu *vcpu)
 	return 1;
 }
 
+static void vmx_disable_shadow_vmcs(struct vcpu_vmx *vmx)
+{
+	vmcs_clear_bits(SECONDARY_VM_EXEC_CONTROL, SECONDARY_EXEC_SHADOW_VMCS);
+	vmcs_write64(VMCS_LINK_POINTER, -1ull);
+}
+
 static inline void nested_release_vmcs12(struct vcpu_vmx *vmx)
 {
 	if (vmx->nested.current_vmptr == -1ull)
@@ -7184,9 +7190,7 @@ static inline void nested_release_vmcs12(struct vcpu_vmx *vmx)
 		   they were modified */
 		copy_shadow_to_vmcs12(vmx);
 		vmx->nested.sync_shadow_vmcs = false;
-		vmcs_clear_bits(SECONDARY_VM_EXEC_CONTROL,
-				SECONDARY_EXEC_SHADOW_VMCS);
-		vmcs_write64(VMCS_LINK_POINTER, -1ull);
+		vmx_disable_shadow_vmcs(vmx);
 	}
 	vmx->nested.posted_intr_nv = -1;
 
@@ -7209,12 +7213,14 @@ static void free_nested(struct vcpu_vmx *vmx)
 
 	vmx->nested.vmxon = false;
 	free_vpid(vmx->nested.vpid02);
-	nested_release_vmcs12(vmx);
+	vmx->nested.posted_intr_nv = -1;
+	vmx->nested.current_vmptr = -1ull;
 	if (vmx->nested.msr_bitmap) {
 		free_page((unsigned long)vmx->nested.msr_bitmap);
 		vmx->nested.msr_bitmap = NULL;
 	}
 	if (enable_shadow_vmcs) {
+		vmx_disable_shadow_vmcs(vmx);
 		vmcs_clear(vmx->vmcs01.shadow_vmcs);
 		free_vmcs(vmx->vmcs01.shadow_vmcs);
 		vmx->vmcs01.shadow_vmcs = NULL;