uio_hv_generic: Enable interrupt for low speed VMBus devices

Hyper-V is adding some "specialty" synthetic devices. Instead of writing
new kernel-level VMBus drivers for these devices, the devices will be
presented to user space via this existing Hyper-V generic UIO driver, so
that a user space driver can handle the device. Since these new synthetic
devices are low speed devices, they don't support monitor bits and we must
use vmbus_setevent() to enable interrupts from the host.

Signed-off-by: Saurabh Sengar <[e-mail]>
Reviewed-by: Long Li <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/uio/uio_hv_generic.c

···
 	dev->channel->inbound.ring_buffer->interrupt_mask = !irq_state;
 	virt_mb();
 
+	if (!dev->channel->offermsg.monitor_allocated && irq_state)
+		vmbus_setevent(dev->channel);
+
 	return 0;
 }
 
···
 	void *ring_buffer;
 	int ret;
 	size_t ring_size = hv_dev_ring_size(channel);
-
-	/* Communicating with host has to be via shared memory not hypercall */
-	if (!channel->offermsg.monitor_allocated) {
-		dev_err(&dev->device, "vmbus channel requires hypercall\n");
-		return -ENOTSUPP;
-	}
 
 	if (!ring_size)
 		ring_size = HV_RING_SIZE * PAGE_SIZE;