PCI: don't allow "pci-phantom=" to mark real devices as phantom functions

IOMMU code mapping / unmapping devices and interrupts will misbehave if
a wrong command line option declared a function "phantom" when there's a
real device at that position. Warn about this and adjust the specified
stride (in the worst case ignoring the option altogether).

Requested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/drivers/passthrough/pci.c

@@ -451,7 +451,24 @@ static struct pci_dev *alloc_pdev(struct pci_seg *pseg, u8 bus, u8 devfn)
                          phantom_devs[i].slot == PCI_SLOT(devfn) &&
                          phantom_devs[i].stride > PCI_FUNC(devfn) )
                     {
-                        pdev->phantom_stride = phantom_devs[i].stride;
+                        pci_sbdf_t sbdf = pdev->sbdf;
+                        unsigned int stride = phantom_devs[i].stride;
+
+                        while ( (sbdf.fn += stride) > PCI_FUNC(devfn) )
+                        {
+                            if ( pci_conf_read16(sbdf, PCI_VENDOR_ID) == 0xffff &&
+                                 pci_conf_read16(sbdf, PCI_DEVICE_ID) == 0xffff )
+                                continue;
+                            stride <<= 1;
+                            printk(XENLOG_WARNING
+                                   "%pp looks to be a real device; bumping %04x:%02x:%02x stride to %u\n",
+                                   &sbdf, phantom_devs[i].seg,
+                                   phantom_devs[i].bus, phantom_devs[i].slot,
+                                   stride);
+                            sbdf = pdev->sbdf;
+                        }
+                        if ( PCI_FUNC(stride) )
+                           pdev->phantom_stride = stride;
                         break;
                     }
             }