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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> 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@redhat.com>
>
> Would it make sense to instead use hv_evmcs_ptr, making the unused
> value -1 instead of 0?

You mean we'll be using:

"hv_evmcs_ptr == 0" meaning "no evmcs" (like now)
"hv_evmcs_ptr == -1" meaing "evmcs not yet mapped" (and
nested_evmcs_is_used() will check for both '0' and '-1')
"hv_evmcs_ptr == anything else" - eVMCS mapped.

Right?

>
> 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)

Or, if we decide to integrate this patch, it'll be:

"hv_evmcs_ptr == -1" meaning "no evmcs" (what is '0' now)
"hv_evmcs_ptr == 0" meaing "evmcs not yet mapped"

I'm not against the idea, hope we won't forget that we have two 'special'
values for 'hv_evmcs_vmptr' and not just one.

>   		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
>

-- 
Vitaly