Re: [PATCH v2] kvm: nVMX: Introduce KVM_CAP_STATE

[David Hildenbrand <david@redhat.com>]

On 09.04.2018 10:37, KarimAllah Ahmed wrote:
> From: Jim Mattson <jmattson@google.com>
> 
> For nested virtualization L0 KVM is managing a bit of state for L2 guests,
> this state can not be captured through the currently available IOCTLs. In
> fact the state captured through all of these IOCTLs is usually a mix of L1
> and L2 state. It is also dependent on whether the L2 guest was running at
> the moment when the process was interrupted to save its state.
> 
> With this capability, there are two new vcpu ioctls: KVM_GET_VMX_STATE and
> KVM_SET_VMX_STATE. These can be used for saving and restoring a VM that is
> in VMX operation.
> 

Very nice work!

>  
> +static int get_vmcs_cache(struct kvm_vcpu *vcpu,
> +			  struct kvm_state __user *user_kvm_state)
> +{
> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +
> +	/*
> +	 * When running L2, the authoritative vmcs12 state is in the
> +	 * vmcs02. When running L1, the authoritative vmcs12 state is
> +	 * in the shadow vmcs linked to vmcs01, unless
> +	 * sync_shadow_vmcs is set, in which case, the authoritative
> +	 * vmcs12 state is in the vmcs12 already.
> +	 */
> +	if (is_guest_mode(vcpu))
> +		sync_vmcs12(vcpu, vmcs12);
> +	else if (enable_shadow_vmcs && !vmx->nested.sync_shadow_vmcs)
> +		copy_shadow_to_vmcs12(vmx);
> +
> +	if (copy_to_user(user_kvm_state->data, vmcs12, sizeof(*vmcs12)))
> +		return -EFAULT;
> +
> +	/*
> +	 * Force a nested exit that guarantees that any state capture
> +	 * afterwards by any IOCTLs (MSRs, etc) will not capture a mix of L1
> +	 * and L2 state.> +	 *

I totally understand why this is nice, I am worried about the
implications. Let's assume migration fails and we want to continue
running the guest on the source. We would now have a "bad" state.

How is this to be handled (e.g. is a SET_STATE necessary?)? I think this
implication should be documented for KVM_GET_STATE.

> +	 * One example where that would lead to an issue is the TSC DEADLINE
> +	 * MSR vs the guest TSC. If the L2 guest is running, the guest TSC will
> +	 * be the L2 TSC while the TSC deadline MSR will contain the L1 TSC
> +	 * deadline MSR. That would lead to a very large (and wrong) "expire"
> +	 * diff when LAPIC is initialized during instance restore (i.e. the
> +	 * instance will appear to have hanged!).
> +	 */
> +	if (is_guest_mode(vcpu))
> +		nested_vmx_vmexit(vcpu, -1, 0, 0);
> +
> +	return 0;
> +}
> +
> +static int get_vmx_state(struct kvm_vcpu *vcpu,
> +			 struct kvm_state __user *user_kvm_state)
> +{
> +	u32 user_data_size;
> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +	struct kvm_state kvm_state = {
> +		.flags = 0,
> +		.format = 0,
> +		.size = sizeof(kvm_state),
> +		.vmx.vmxon_pa = -1ull,
> +		.vmx.vmcs_pa = -1ull,
> +	};
> +
> +	if (copy_from_user(&user_data_size, &user_kvm_state->size,
> +			   sizeof(user_data_size)))
> +		return -EFAULT;
> +
> +	if (nested_vmx_allowed(vcpu) && vmx->nested.vmxon) {
> +		kvm_state.vmx.vmxon_pa = vmx->nested.vmxon_ptr;
> +		kvm_state.vmx.vmcs_pa = vmx->nested.current_vmptr;
> +
> +		if (vmx->nested.current_vmptr != -1ull)
> +			kvm_state.size += VMCS12_SIZE;
> +
> +		if (is_guest_mode(vcpu)) {
> +			kvm_state.flags |= KVM_STATE_GUEST_MODE;
> +
> +			if (vmx->nested.nested_run_pending)
> +				kvm_state.flags |= KVM_STATE_RUN_PENDING;
> +		}
> +	}
> +
> +	if (user_data_size < kvm_state.size) {
> +		if (copy_to_user(&user_kvm_state->size, &kvm_state.size,
> +				 sizeof(kvm_state.size)))
> +			return -EFAULT;
> +		return -E2BIG;
> +	}
> +
> +	if (copy_to_user(user_kvm_state, &kvm_state, sizeof(kvm_state)))
> +		return -EFAULT;
> +
> +	if (vmx->nested.current_vmptr == -1ull)
> +		return 0;
> +
> +	return get_vmcs_cache(vcpu, user_kvm_state);
> +}
> +
> +static int set_vmcs_cache(struct kvm_vcpu *vcpu,
> +			  struct kvm_state __user *user_kvm_state,
> +			  struct kvm_state *kvm_state)
> +
> +{
> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +	u32 exit_qual;
> +	int ret;
> +
> +	if ((kvm_state->size < (sizeof(*vmcs12) + sizeof(*kvm_state))) ||
> +	    kvm_state->vmx.vmcs_pa == kvm_state->vmx.vmxon_pa ||
> +	    !page_address_valid(vcpu, kvm_state->vmx.vmcs_pa))
> +		return -EINVAL;
> +
> +	if (copy_from_user(vmcs12, user_kvm_state->data, sizeof(*vmcs12)))
> +		return -EFAULT;
> +
> +	if (vmcs12->revision_id != VMCS12_REVISION)
> +		return -EINVAL;
> +
> +	set_current_vmptr(vmx, kvm_state->vmx.vmcs_pa);
> +
> +	if (!(kvm_state->flags & KVM_STATE_GUEST_MODE))
> +		return 0;
> +
> +	if (check_vmentry_prereqs(vcpu, vmcs12) ||
> +	    check_vmentry_postreqs(vcpu, vmcs12, &exit_qual))
> +		return -EINVAL;
> +
> +	ret = enter_vmx_non_root_mode(vcpu, true);
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * This request will result in a call to
> +	 * nested_get_vmcs12_pages before the next VM-entry.
> +	 */
> +	kvm_make_request(KVM_REQ_GET_VMCS12_PAGES, vcpu);

Can you elaborate (+document) why this is needed instead of trying to
get the page right away?

Thanks!

-- 

Thanks,

David / dhildenb