x86/kvm/hyper-v: inject #GP only when invalid SINTx vector is unmasked

Hyper-V 2016 on KVM with SynIC enabled doesn't boot with the following
trace:

    kvm_entry:            vcpu 0
    kvm_exit:             reason MSR_WRITE rip 0xfffff8000131c1e5 info 0 0
    kvm_hv_synic_set_msr: vcpu_id 0 msr 0x40000090 data 0x10000 host 0
    kvm_msr:              msr_write 40000090 = 0x10000 (#GP)
    kvm_inj_exception:    #GP (0x0)

KVM acts according to the following statement from TLFS:

"
11.8.4 SINTx Registers
...
Valid values for vector are 16-255 inclusive. Specifying an invalid
vector number results in #GP.
"

However, I checked and genuine Hyper-V doesn't #GP when we write 0x10000
to SINTx. I checked with Microsoft and they confirmed that if either the
Masked bit (bit 16) or the Polling bit (bit 18) is set to 1, then they
ignore the value of Vector. Make KVM act accordingly.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/hyperv.c

@@ -96,9 +96,17 @@ static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint,
 			  u64 data, bool host)
 {
 	int vector, old_vector;
+	bool masked;
 
 	vector = data & HV_SYNIC_SINT_VECTOR_MASK;
-	if (vector < HV_SYNIC_FIRST_VALID_VECTOR && !host)
+	masked = data & HV_SYNIC_SINT_MASKED;
+
+	/*
+	 * Valid vectors are 16-255, however, nested Hyper-V attempts to write
+	 * default '0x10000' value on boot and this should not #GP. We need to
+	 * allow zero-initing the register from host as well.
+	 */
+	if (vector < HV_SYNIC_FIRST_VALID_VECTOR && !host && !masked)
 		return 1;
 	/*
 	 * Guest may configure multiple SINTs to use the same vector, so