PCI: hv: Make sure the bus domain is really unique

When Linux runs as a guest VM in Hyper-V and Hyper-V adds the virtual PCI
bus to the guest, Hyper-V always provides unique PCI domain.

commit 4a9b0933 ("PCI: hv: Use device serial number as PCI domain")
overrode unique domain with the serial number of the first device added to
the virtual PCI bus.

The reason for that patch was to have a consistent and short name for the
device, but Hyper-V doesn't provide unique serial numbers. Using non-unique
serial numbers as domain IDs leads to duplicate device addresses, which
causes PCI bus registration to fail.

commit 0c195567 ("netvsc: transparent VF management") avoids the need
for commit 4a9b0933 ("PCI: hv: Use device serial number as PCI
domain").  When scripts were used to configure VF devices, the name of
the VF needed to be consistent and short, but with commit 0c195567
("netvsc: transparent VF management") all the setup is done in the kernel,
and we do not need to maintain consistent name.

Revert commit 4a9b0933 ("PCI: hv: Use device serial number as PCI
domain") so we can reliably support multiple devices being assigned to
a guest.

Tag the patch for stable kernels containing commit 0c195567
("netvsc: transparent VF management").

Fixes: 4a9b0933 ("PCI: hv: Use device serial number as PCI domain")
Signed-off-by: Sridhar Pitchai <sridhar.pitchai@microsoft.com>
[lorenzo.pieralisi@arm.com: trimmed commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org # v4.14+
Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/host/pci-hyperv.c

@@ -1575,17 +1575,6 @@ static struct hv_pci_dev *new_pcichild_device(struct hv_pcibus_device *hbus,
 	get_pcichild(hpdev, hv_pcidev_ref_childlist);
 	spin_lock_irqsave(&hbus->device_list_lock, flags);
 
-	/*
-	 * When a device is being added to the bus, we set the PCI domain
-	 * number to be the device serial number, which is non-zero and
-	 * unique on the same VM.  The serial numbers start with 1, and
-	 * increase by 1 for each device.  So device names including this
-	 * can have shorter names than based on the bus instance UUID.
-	 * Only the first device serial number is used for domain, so the
-	 * domain number will not change after the first device is added.
-	 */
-	if (list_empty(&hbus->children))
-		hbus->sysdata.domain = desc->ser;
 	list_add_tail(&hpdev->list_entry, &hbus->children);
 	spin_unlock_irqrestore(&hbus->device_list_lock, flags);
 	return hpdev;