KVM: x86: Set kvm_x86_ops only after ->hardware_setup() completes

Set kvm_x86_ops with the vendor's ops only after ->hardware_setup()
completes to "prevent" using kvm_x86_ops before they are ready, i.e. to
generate a null pointer fault instead of silently consuming unconfigured
state.

An alternative implementation would be to have ->hardware_setup()
return the vendor's ops, but that would require non-trivial refactoring,
and would arguably result in less readable code, e.g. ->hardware_setup()
would need to use ERR_PTR() in multiple locations, and each vendor's
declaration of the runtime ops would be less obvious.

No functional change intended.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200321202603.19355-6-sean.j.christopherson@intel.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -7359,8 +7359,6 @@ int kvm_arch_init(void *opaque)
 	if (r)
 		goto out_free_percpu;
 
-	kvm_x86_ops = ops->runtime_ops;
-
 	kvm_mmu_set_mask_ptes(PT_USER_MASK, PT_ACCESSED_MASK,
 			PT_DIRTY_MASK, PT64_NX_MASK, 0,
 			PT_PRESENT_MASK, 0, sme_me_mask);
@@ -9640,6 +9638,8 @@ int kvm_arch_hardware_setup(void *opaque)
 	if (r != 0)
 		return r;
 
+	kvm_x86_ops = ops->runtime_ops;
+
 	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
 		supported_xss = 0;