PCI: ARI is a PCIe v2 feature

The function pci_enable_ari() may mistakenly set the downstream port
of a v1 PCIe switch in ARI Forwarding mode.  This is a PCIe v2 feature,
and with an SR-IOV device on that switch port believing the switch above
is ARI capable it may attempt to use functions 8-255, translating into
invalid (non-zero) device numbers for that bus.  This has been seen
to cause Completion Timeouts and general misbehaviour including hangs
and panics.

Cc: stable@kernel.org
Acked-by: Don Dutile <ddutile@redhat.com>
Tested-by: Don Dutile <ddutile@redhat.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/pci/pci.c

@@ -1905,7 +1905,7 @@ void pci_enable_ari(struct pci_dev *dev)
 {
 	int pos;
 	u32 cap;
-	u16 ctrl;
+	u16 flags, ctrl;
 	struct pci_dev *bridge;
 
 	if (!pci_is_pcie(dev) || dev->devfn)
@@ -1923,6 +1923,11 @@ void pci_enable_ari(struct pci_dev *dev)
 	if (!pos)
 		return;
 
+	/* ARI is a PCIe v2 feature */
+	pci_read_config_word(bridge, pos + PCI_EXP_FLAGS, &flags);
+	if ((flags & PCI_EXP_FLAGS_VERS) < 2)
+		return;
+
 	pci_read_config_dword(bridge, pos + PCI_EXP_DEVCAP2, &cap);
 	if (!(cap & PCI_EXP_DEVCAP2_ARI))
 		return;