Commit 0a701aa6 authored Apr 10, 2017 by Yongji Xie Committed by Bjorn Helgaas Apr 19, 2017

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>

parent c9c75143

drivers/pci/pci.c

+8 −2

Original line number	Diff line number	Diff line
		@@ -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;
		}

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