Re: [PATCH v2 1/7] KVM: nVMX: Introduce nested_evmcs_is_used()

[Paolo Bonzini]

On 17/05/21 15:50, Vitaly Kuznetsov wrote:
> Unlike regular set_current_vmptr(), nested_vmx_handle_enlightened_vmptrld()
> can not be called directly from vmx_set_nested_state() as KVM may not have
> all the information yet (e.g. HV_X64_MSR_VP_ASSIST_PAGE MSR may not be
> restored yet). Enlightened VMCS is mapped later while getting nested state
> pages. In the meantime, vmx->nested.hv_evmcs remains NULL and using it
> for various checks is incorrect. In particular, if KVM_GET_NESTED_STATE is
> called right after KVM_SET_NESTED_STATE, KVM_STATE_NESTED_EVMCS flag in the
> resulting state will be unset (and such state will later fail to load).
>
> Introduce nested_evmcs_is_used() and use 'is_guest_mode(vcpu) &&
> vmx->nested.current_vmptr == -1ull' check to detect not-yet-mapped eVMCS
> after restore.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

Would it make sense to instead use hv_evmcs_ptr, making the unused
value -1 instead of 0?

I even had this untested patch already lying around as a cleanup I
never bothered to submit:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index a19cfcb625da..dd3e4ddaaf26 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -187,7 +187,7 @@ static int nested_vmx_fail(struct kvm_vcpu *vcpu, u32 vm_instruction_error)
 	 * failValid writes the error number to the current VMCS, which
 	 * can't be done if there isn't a current VMCS.
 	 */
-	if (vmx->nested.current_vmptr == -1ull && !vmx->nested.hv_evmcs)
+	if (vmx->nested.current_vmptr == -1ull && vmx->nested.hv_evmcs_vmptr == -1)
 		return nested_vmx_failInvalid(vcpu);
 
 	return nested_vmx_failValid(vcpu, vm_instruction_error);
@@ -221,11 +221,11 @@ static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	if (!vmx->nested.hv_evmcs)
+	if (vmx->nested.hv_evmcs_vmptr == -1)
 		return;
 
 	kvm_vcpu_unmap(vcpu, &vmx->nested.hv_evmcs_map, NULL, true);
-	vmx->nested.hv_evmcs_vmptr = 0;
+	vmx->nested.hv_evmcs_vmptr = -1;
 	vmx->nested.hv_evmcs = NULL;
 }
 
@@ -1978,10 +1978,8 @@ static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
 	if (!nested_enlightened_vmentry(vcpu, &evmcs_gpa))
 		return EVMPTRLD_DISABLED;
 
-	if (unlikely(!vmx->nested.hv_evmcs ||
-		     evmcs_gpa != vmx->nested.hv_evmcs_vmptr)) {
-		if (!vmx->nested.hv_evmcs)
-			vmx->nested.current_vmptr = -1ull;
+	if (unlikely(evmcs_gpa != vmx->nested.hv_evmcs_vmptr)) {
+		vmx->nested.current_vmptr = -1ull;
 
 		nested_release_evmcs(vcpu);
 
@@ -2052,7 +2050,7 @@ void nested_sync_vmcs12_to_shadow(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	if (vmx->nested.hv_evmcs) {
+	if (vmx->nested.hv_evmcs_vmptr != -1) {
 		copy_vmcs12_to_enlightened(vmx);
 		/* All fields are clean */
 		vmx->nested.hv_evmcs->hv_clean_fields |=
@@ -2204,7 +2202,7 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
 	u32 exec_control;
 	u64 guest_efer = nested_vmx_calc_efer(vmx, vmcs12);
 
-	if (vmx->nested.dirty_vmcs12 || vmx->nested.hv_evmcs)
+	if (vmx->nested.dirty_vmcs12 || vmx->nested.hv_evmcs_vmptr != -1)
 		prepare_vmcs02_early_rare(vmx, vmcs12);
 
 	/*
@@ -2487,9 +2485,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 			  enum vm_entry_failure_code *entry_failure_code)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
-	struct hv_enlightened_vmcs *hv_evmcs = vmx->nested.hv_evmcs;
 
-	if (vmx->nested.dirty_vmcs12 || hv_evmcs) {
+	if (vmx->nested.dirty_vmcs12 || vmx->nested.hv_evmcs_vmptr != -1) {
 		prepare_vmcs02_rare(vmx, vmcs12);
 		vmx->nested.dirty_vmcs12 = false;
 	}
@@ -3075,7 +3072,7 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
 	 * L2 was running), map it here to make sure vmcs12 changes are
 	 * properly reflected.
 	 */
-	if (vmx->nested.enlightened_vmcs_enabled && !vmx->nested.hv_evmcs) {
+	if (vmx->nested.enlightened_vmcs_enabled && vmx->nested.hv_evmcs_vmptr == -1) {
 		enum nested_evmptrld_status evmptrld_status =
 			nested_vmx_handle_enlightened_vmptrld(vcpu, false);
 
@@ -3419,7 +3416,7 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
 
 	load_vmcs12_host_state(vcpu, vmcs12);
 	vmcs12->vm_exit_reason = exit_reason.full;
-	if (enable_shadow_vmcs || vmx->nested.hv_evmcs)
+	if (enable_shadow_vmcs || vmx->nested.hv_evmcs_vmptr != -1)
 		vmx->nested.need_vmcs12_to_shadow_sync = true;
 	return NVMX_VMENTRY_VMEXIT;
 }
@@ -3449,7 +3446,7 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
 		return nested_vmx_failInvalid(vcpu);
 	}
 
-	if (CC(!vmx->nested.hv_evmcs && vmx->nested.current_vmptr == -1ull))
+	if (CC(vmx->nested.hv_evmcs_vmptr == -1 && vmx->nested.current_vmptr == -1ull))
 		return nested_vmx_failInvalid(vcpu);
 
 	vmcs12 = get_vmcs12(vcpu);
@@ -3463,7 +3460,7 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
 	if (CC(vmcs12->hdr.shadow_vmcs))
 		return nested_vmx_failInvalid(vcpu);
 
-	if (vmx->nested.hv_evmcs) {
+	if (vmx->nested.hv_evmcs_vmptr != -1) {
 		copy_enlightened_to_vmcs12(vmx);
 		/* Enlightened VMCS doesn't have launch state */
 		vmcs12->launch_state = !launch;
@@ -4014,10 +4011,10 @@ static void sync_vmcs02_to_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	if (vmx->nested.hv_evmcs)
+	if (vmx->nested.hv_evmcs_vmptr != -1)
 		sync_vmcs02_to_vmcs12_rare(vcpu, vmcs12);
 
-	vmx->nested.need_sync_vmcs02_to_vmcs12_rare = !vmx->nested.hv_evmcs;
+	vmx->nested.need_sync_vmcs02_to_vmcs12_rare = (vmx->nested.hv_evmcs_vmptr == -1);
 
 	vmcs12->guest_cr0 = vmcs12_guest_cr0(vcpu, vmcs12);
 	vmcs12->guest_cr4 = vmcs12_guest_cr4(vcpu, vmcs12);
@@ -4514,7 +4511,7 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 	}
 
 	if ((vm_exit_reason != -1) &&
-	    (enable_shadow_vmcs || vmx->nested.hv_evmcs))
+	    (enable_shadow_vmcs || vmx->nested.hv_evmcs_vmptr != -1))
 		vmx->nested.need_vmcs12_to_shadow_sync = true;
 
 	/* in case we halted in L2 */
@@ -5210,7 +5207,7 @@ static int handle_vmptrld(struct kvm_vcpu *vcpu)
 		return nested_vmx_fail(vcpu, VMXERR_VMPTRLD_VMXON_POINTER);
 
 	/* Forbid normal VMPTRLD if Enlightened version was used */
-	if (vmx->nested.hv_evmcs)
+	if (vmx->nested.hv_evmcs_vmptr != -1)
 		return 1;
 
 	if (vmx->nested.current_vmptr != vmptr) {
@@ -5266,7 +5263,7 @@ static int handle_vmptrst(struct kvm_vcpu *vcpu)
 	if (!nested_vmx_check_permission(vcpu))
 		return 1;
 
-	if (unlikely(to_vmx(vcpu)->nested.hv_evmcs))
+	if (unlikely(to_vmx(vcpu)->nested.hv_evmcs_vmptr != -1))
 		return 1;
 
 	if (get_vmx_mem_address(vcpu, exit_qual, instr_info,
@@ -6038,7 +6035,7 @@ static int vmx_get_nested_state(struct kvm_vcpu *vcpu,
 		if (vmx_has_valid_vmcs12(vcpu)) {
 			kvm_state.size += sizeof(user_vmx_nested_state->vmcs12);
 
-			if (vmx->nested.hv_evmcs)
+			if (vmx->nested.hv_evmcs_vmptr != -1)
 				kvm_state.flags |= KVM_STATE_NESTED_EVMCS;
 
 			if (is_guest_mode(vcpu) &&
@@ -6094,7 +6091,7 @@ static int vmx_get_nested_state(struct kvm_vcpu *vcpu,
 	} else  {
 		copy_vmcs02_to_vmcs12_rare(vcpu, get_vmcs12(vcpu));
 		if (!vmx->nested.need_vmcs12_to_shadow_sync) {
-			if (vmx->nested.hv_evmcs)
+			if (vmx->nested.hv_evmcs_vmptr != -1)
 				copy_enlightened_to_vmcs12(vmx);
 			else if (enable_shadow_vmcs)
 				copy_shadow_to_vmcs12(vmx);
diff --git a/arch/x86/kvm/vmx/nested.h b/arch/x86/kvm/vmx/nested.h
index 184418baeb3c..55925be48973 100644
--- a/arch/x86/kvm/vmx/nested.h
+++ b/arch/x86/kvm/vmx/nested.h
@@ -63,7 +63,7 @@ static inline int vmx_has_valid_vmcs12(struct kvm_vcpu *vcpu)
 	 * have vmcs12 if it is true.
 	 */
 	return is_guest_mode(vcpu) || vmx->nested.current_vmptr != -1ull ||
-		vmx->nested.hv_evmcs;
+		vmx->nested.hv_evmcs_vmptr != -1;
 }
 
 static inline u16 nested_get_vpid02(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index f2fd447eed45..33208aa4ac87 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6974,6 +6974,7 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
 
 	vmx->nested.posted_intr_nv = -1;
 	vmx->nested.current_vmptr = -1ull;
+	vmx->nested.hv_evmcs_vmptr = -1;
 
 	vcpu->arch.microcode_version = 0x100000000ULL;
 	vmx->msr_ia32_feature_control_valid_bits = FEAT_CTL_LOCKED;

Paolo