KVM: x86: hyper-v: Check access to HVCALL_NOTIFY_LONG_SPIN_WAIT hypercall

TLFS6.0b states that partition issuing HVCALL_NOTIFY_LONG_SPIN_WAIT must
posess 'UseHypercallForLongSpinWait' privilege but there's no
corresponding feature bit. Instead, we have "Recommended number of attempts
to retry a spinlock failure before notifying the hypervisor about the
failures. 0xFFFFFFFF indicates never notify." Use this to check access to
the hypercall. Also, check against zero as the corresponding CPUID must
be set (and '0' attempts before re-try is weird anyway).
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210521095204.2161214-22-vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -2092,6 +2092,17 @@ static void kvm_hv_hypercall_read_xmm(struct kvm_hv_hcall *hc)
 
 static bool hv_check_hypercall_access(struct kvm_vcpu_hv *hv_vcpu, u16 code)
 {
+	if (!hv_vcpu->enforce_cpuid)
+		return true;
+
+	switch (code) {
+	case HVCALL_NOTIFY_LONG_SPIN_WAIT:
+		return hv_vcpu->cpuid_cache.enlightenments_ebx &&
+			hv_vcpu->cpuid_cache.enlightenments_ebx != U32_MAX;
+	default:
+		break;
+	}
+
 	return true;
 }