KVM: nVMX: stop abusing need_vmcs12_to_shadow_sync for eVMCS mapping

When vmx_set_nested_state() happens, we may not have all the required
data to map enlightened VMCS: e.g. HV_X64_MSR_VP_ASSIST_PAGE MSR may not
yet be restored so we need a postponed action. Currently, we (ab)use
need_vmcs12_to_shadow_sync/nested_sync_vmcs12_to_shadow() for that but
this is not ideal:
- We may not need to sync anything if L2 is running
- It is hard to propagate errors from nested_sync_vmcs12_to_shadow()
 as we call it from vmx_prepare_switch_to_guest() which happens just
 before we do VMLAUNCH, the code is not ready to handle errors there.

Move eVMCS mapping to nested_get_vmcs12_pages() and request
KVM_REQ_GET_VMCS12_PAGES, it seems to be is less abusive in nature.
It would probably be possible to introduce a specialized KVM_REQ_EVMCS_MAP
but it is undesirable to propagate eVMCS specifics all the way up to x86.c

Note, we don't need to request KVM_REQ_GET_VMCS12_PAGES from
vmx_set_nested_state() directly as nested_vmx_enter_non_root_mode() already
does that. Requesting KVM_REQ_GET_VMCS12_PAGES is done to document the
(non-obvious) side-effect and to be future proof.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -1996,14 +1996,6 @@ void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	/*
-	 * hv_evmcs may end up being not mapped after migration (when
-	 * L2 was running), map it here to make sure vmcs12 changes are
-	 * properly reflected.
-	 */
-	if (vmx->nested.enlightened_vmcs_enabled && !vmx->nested.hv_evmcs)
-		nested_vmx_handle_enlightened_vmptrld(vcpu, false);
-
 	if (vmx->nested.hv_evmcs) {
 		copy_vmcs12_to_enlightened(vmx);
 		/* All fields are clean */
@@ -3062,6 +3054,14 @@ static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
 	struct page *page;
 	u64 hpa;
 
+	/*
+	 * hv_evmcs may end up being not mapped after migration (when
+	 * L2 was running), map it here to make sure vmcs12 changes are
+	 * properly reflected.
+	 */
+	if (vmx->nested.enlightened_vmcs_enabled && !vmx->nested.hv_evmcs)
+		nested_vmx_handle_enlightened_vmptrld(vcpu, false);
+
 	if (nested_cpu_has2(vmcs12, SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES)) {
 		/*
 		 * Translate L1 physical address to host physical
@@ -5904,10 +5904,12 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
 		set_current_vmptr(vmx, kvm_state->hdr.vmx.vmcs12_pa);
 	} else if (kvm_state->flags & KVM_STATE_NESTED_EVMCS) {
 		/*
-		 * Sync eVMCS upon entry as we may not have
-		 * HV_X64_MSR_VP_ASSIST_PAGE set up yet.
+		 * nested_vmx_handle_enlightened_vmptrld() cannot be called
+		 * directly from here as HV_X64_MSR_VP_ASSIST_PAGE may not be
+		 * restored yet. EVMCS will be mapped from
+		 * nested_get_vmcs12_pages().
 		 */
-		vmx->nested.need_vmcs12_to_shadow_sync = true;
+		kvm_make_request(KVM_REQ_GET_VMCS12_PAGES, vcpu);
 	} else {
 		return -EINVAL;
 	}