Drivers: hv: vmbus: Export hv_vmbus_exists() and use it in pci-hyperv

With commit f84b21da36 ("PCI: hv: Don't load the driver for baremetal root partition"),
the bare metal Linux root partition won't use the pci-hyperv driver, but
when a Linux VM runs on the Linux root partition, pci-hyperv's module_init
function init_hv_pci_drv() can still run, e.g. in the case of
CONFIG_PCI_HYPERV=y, even if the VMBus driver is not used in such a VM
(i.e. the hv_vmbus driver's init function returns -ENODEV due to
vmbus_root_device being NULL).

In such a Linux VM, init_hv_pci_drv() runs with a side effect: the 3
hvpci_block_ops callbacks are set to functions that depend on hv_vmbus.

Later, when the MLX driver in such a VM invokes the callbacks, e.g. in
drivers/net/ethernet/mellanox/mlx5/core/lib/hv.c:
mlx5_hv_register_invalidate(), hvpci_block_ops.reg_blk_invalidate() is
hv_register_block_invalidate() rather than a NULL function pointer, and
hv_register_block_invalidate() assumes that it can find a struct
hv_pcibus_device from pdev->bus->sysdata, which is false in such a VM.

Consequently, hv_register_block_invalidate() -> get_pcichild_wslot() ->
spin_lock_irqsave() may hang since it can be accessing an invalid
spinlock pointer.

Fix the issue by exporting hv_vmbus_exists() and using it in pci-hyperv:

    hv_root_partition() is true and hv_nested is false ==>
	hv_vmbus_exists() is false.

    hv_root_partition() is true and hv_nested is true ==>
	hv_vmbus_exists() is true.

    hv_root_partition() is false ==> hv_vmbus_exists() is true.

While at it, rename vmbus_exists() to hv_vmbus_exists() to follow the
convention that all public functions have the hv_ prefix; also change
the return value's type from int to bool to make the code more readable;
also move the two pr_info() calls.

Reported-by: Mukesh Rathor <mrathor@linux.microsoft.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/vmbus_drv.c

@@ -101,13 +101,11 @@ struct device *hv_get_vmbus_root_device(void)
 }
 EXPORT_SYMBOL_GPL(hv_get_vmbus_root_device);
 
-static int vmbus_exists(void)
+bool hv_vmbus_exists(void)
 {
-	if (vmbus_root_device == NULL)
-		return -ENODEV;
-
-	return 0;
+	return vmbus_root_device != NULL;
 }
+EXPORT_SYMBOL_GPL(hv_vmbus_exists);
 
 static u8 channel_monitor_group(const struct vmbus_channel *channel)
 {
@@ -1577,11 +1575,10 @@ int __vmbus_driver_register(struct hv_driver *hv_driver, struct module *owner, c
 {
 	int ret;
 
-	pr_info("registering driver %s\n", hv_driver->name);
+	if (!hv_vmbus_exists())
+		return -ENODEV;
 
-	ret = vmbus_exists();
-	if (ret < 0)
-		return ret;
+	pr_info("registering driver %s\n", hv_driver->name);
 
 	hv_driver->driver.name = hv_driver->name;
 	hv_driver->driver.owner = owner;
@@ -1607,9 +1604,8 @@ EXPORT_SYMBOL_GPL(__vmbus_driver_register);
  */
 void vmbus_driver_unregister(struct hv_driver *hv_driver)
 {
-	pr_info("unregistering driver %s\n", hv_driver->name);
-
-	if (!vmbus_exists()) {
+	if (hv_vmbus_exists()) {
+		pr_info("unregistering driver %s\n", hv_driver->name);
 		driver_unregister(&hv_driver->driver);
 		vmbus_free_dynids(hv_driver);
 	}

drivers/pci/controller/pci-hyperv.c

@@ -4172,7 +4172,7 @@ static int __init init_hv_pci_drv(void)
 	if (!hv_is_hyperv_initialized())
 		return -ENODEV;
 
-	if (hv_root_partition() && !hv_nested)
+	if (!hv_vmbus_exists())
 		return -ENODEV;
 
 	ret = hv_pci_irqchip_init();

include/linux/hyperv.h

@@ -1304,6 +1304,8 @@ static inline void *hv_get_drvdata(struct hv_device *dev)
 
 struct device *hv_get_vmbus_root_device(void);
 
+bool hv_vmbus_exists(void);
+
 struct hv_ring_buffer_debug_info {
 	u32 current_interrupt_mask;
 	u32 current_read_index;