Refuse to load KVM if NX support is not available.  Shadow paging has
assumed NX support since commit 9167ab79 ("KVM: vmx, svm: always run
with EFER.NXE=1 when shadow paging is active"), and NPT has assumed NX
support since commit b8e8c830 ("kvm: mmu: ITLB_MULTIHIT mitigation").
While the NX huge pages mitigation should not be enabled by default for
AMD CPUs, it can be turned on by userspace at will.

Unlike Intel CPUs, AMD does not provide a way for firmware to disable NX
support, and Linux always sets EFER.NX=1 if it is supported.  Given that
it's extremely unlikely that a CPU supports NPT but not NX, making NX a
formal requirement is far simpler than adding requirements to the
mitigation flow.

Fixes: 9167ab79 ("KVM: vmx, svm: always run with EFER.NXE=1 when shadow paging is active")
Fixes: b8e8c830 ("kvm: mmu: ITLB_MULTIHIT mitigation")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Message-Id: <20210615164535.2146172-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>