PCI: Add pcibios_default_alignment() for arch-specific alignment control

When VFIO passes through a PCI device to a guest, it does not allow the
guest to mmap BARs that are smaller than PAGE_SIZE unless it can reserve
the rest of the page (see vfio_pci_probe_mmaps()). This is because a page
might contain several small BARs for unrelated devices and a guest should
not be able to access all of them.

VFIO emulates guest accesses to non-mappable BARs, which is functional but
slow. On systems with large page sizes, e.g., PowerNV with 64K pages, BARs
are more likely to share a page and performance is more likely to be a
problem.

Add a weak function to set default alignment for all PCI devices.  An arch
can override it to force the PCI core to place memory BARs on their own
pages.
Signed-off-by: Yongji Xie <elohimes@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci.c

@@ -4947,6 +4947,11 @@ void pci_ignore_hotplug(struct pci_dev *dev)
 }
 EXPORT_SYMBOL_GPL(pci_ignore_hotplug);
 
+resource_size_t __weak pcibios_default_alignment(void)
+{
+	return 0;
+}
+
 #define RESOURCE_ALIGNMENT_PARAM_SIZE COMMAND_LINE_SIZE
 static char resource_alignment_param[RESOURCE_ALIGNMENT_PARAM_SIZE] = {0};
 static DEFINE_SPINLOCK(resource_alignment_lock);
@@ -4962,14 +4967,15 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev)
 {
 	int seg, bus, slot, func, align_order, count;
 	unsigned short vendor, device, subsystem_vendor, subsystem_device;
-	resource_size_t align = 0;
+	resource_size_t align = pcibios_default_alignment();
 	char *p;
 
 	spin_lock(&resource_alignment_lock);
 	p = resource_alignment_param;
-	if (!*p)
+	if (!*p && !align)
 		goto out;
 	if (pci_has_flag(PCI_PROBE_ONLY)) {
+		align = 0;
 		pr_info_once("PCI: Ignoring requested alignments (PCI_PROBE_ONLY)\n");
 		goto out;
 	}