On 29 May 2012, Avi Kivity wrote on the KVM mailing list:

It looks like the lines

        vmx_set_cr0(vcpu, vmcs12->guest_cr0);
        vmcs_writel(CR0_READ_SHADOW, nested_read_cr0(vmcs12));

        vmx_set_cr4(vcpu, vmcs12->guest_cr4);
        vmcs_writel(CR4_READ_SHADOW, nested_read_cr4(vmcs12));


in prepare_vmcs02() are too dangerous.  They only let the hardware check
cr4, but not software, so a future cr4 bit which is not handled
correctly by kvm can be set in a nested guest. L1 appears to be
protected, we use kvm_set_cr4().