Revert "KVM: x86: WARN and reject loading KVM if NX is supported but not enabled"

Let KVM load if EFER.NX=0 even if NX is supported, the analysis and
testing (or lack thereof) for the non-PAE host case was garbage.

If the kernel won't be using PAE paging, .Ldefault_entry in head_32.S
skips over the entire EFER sequence.  Hopefully that can be changed in
the future to allow KVM to require EFER.NX, but the motivation behind
KVM's requirement isn't yet merged.  Reverting and revisiting the mess
at a later date is by far the safest approach.

This reverts commit 8bbed95d.

Fixes: 8bbed95d ("KVM: x86: WARN and reject loading KVM if NX is supported but not enabled")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210625001853.318148-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -10981,9 +10981,6 @@ int kvm_arch_hardware_setup(void *opaque)
 	int r;
 
 	rdmsrl_safe(MSR_EFER, &host_efer);
-	if (WARN_ON_ONCE(boot_cpu_has(X86_FEATURE_NX) &&
-			 !(host_efer & EFER_NX)))
-		return -EIO;
 
 	if (boot_cpu_has(X86_FEATURE_XSAVES))
 		rdmsrl(MSR_IA32_XSS, host_xss);