Re: [PATCH 6/7] KVM: nVMX: don't load PDPTRS right after nested state set

[Sean Christopherson <seanjc@google.com>]

On Wed, Feb 17, 2021, Maxim Levitsky wrote:
> Just like all other nested memory accesses, after a migration loading
> PDPTRs should be delayed to first VM entry to ensure
> that guest memory is fully initialized.
> 
> Just move the call to nested_vmx_load_cr3 to nested_get_vmcs12_pages
> to implement this.

I don't love this approach.  KVM_SET_NESTED_STATE will now succeed with a bad
vmcs12.GUEST_CR3.  At a minimum, GUEST_CR3 should be checked in
nested_vmx_check_guest_state().  It also feels like vcpu->arch.cr3 should be set
immediately, e.g. KVM_SET_NESTED_STATE -> KVM_GET_SREGS should reflect L2's CR3
even if KVM_RUN hasn't been invoked.

> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index f9de729dbea6..26084f8eee82 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2596,11 +2596,6 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>  		return -EINVAL;
>  	}
>  
> -	/* Shadow page tables on either EPT or shadow page tables. */
> -	if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3, nested_cpu_has_ept(vmcs12),
> -				entry_failure_code))
> -		return -EINVAL;
> -
>  	/*
>  	 * Immediately write vmcs02.GUEST_CR3.  It will be propagated to vmcs12
>  	 * on nested VM-Exit, which can occur without actually running L2 and
> @@ -3138,11 +3133,16 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
>  static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
>  {
>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +	enum vm_entry_failure_code entry_failure_code;
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  	struct kvm_host_map *map;
>  	struct page *page;
>  	u64 hpa;
>  
> +	if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3, nested_cpu_has_ept(vmcs12),
> +				&entry_failure_code))
> +		return false;
> +
>  	if (nested_cpu_has2(vmcs12, SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES)) {
>  		/*
>  		 * Translate L1 physical address to host physical
> @@ -3386,6 +3386,10 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
>  	}
>  
>  	if (from_vmentry) {
> +		if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3,
> +		    nested_cpu_has_ept(vmcs12), &entry_failure_code))
> +			goto vmentry_fail_vmexit_guest_mode;

This neglects to set both exit_reason.basic and vmcs12->exit_qualification.

> +
>  		failed_index = nested_vmx_load_msr(vcpu,
>  						   vmcs12->vm_entry_msr_load_addr,
>  						   vmcs12->vm_entry_msr_load_count);
> -- 
> 2.26.2
>