[BUG ? ] Each pci bridge only supports hotplugging 16 numbers of virtio-blk/virtio-net devices

[Hailiang Zhang]

Hi,

We tried to hot add more than 16 numbers of virtio-blk devices to pci bridge, but found that only 16 of them are available in VM.

There are âno spaceâ error messages in dmesg:

[    4.666106] pci 0000:00:03.0: PCI bridge to [bus 01]

[    4.666191] pci 0000:00:03.0:   bridge window [io  0x7000-0x7fff]

[    4.670044] pci 0000:00:03.0:   bridge window [mem 0xfe800000-0xfe9fffff]

[    4.672650] pci 0000:00:03.0:   bridge window [mem 0xfcc00000-0xfcdfffff 64bit pref]

[    4.677876] pci 0000:00:07.0: PCI bridge to [bus 02]

[    4.677967] pci 0000:00:07.0:   bridge window [io  0x6000-0x6fff]

[    4.681816] pci 0000:00:07.0:   bridge window [mem 0xfe600000-0xfe7fffff]

[    4.684422] pci 0000:00:07.0:   bridge window [mem 0xfca00000-0xfcbfffff 64bit pref]

â â

[   85.779103] pci 0000:02:17.0: [1af4:1001] type 00 class 0x010000

[   85.779194] pci 0000:02:17.0: reg 0x10: [io  0x0000-0x003f]

[   85.779235] pci 0000:02:17.0: reg 0x14: [mem 0x00000000-0x00000fff]

[   85.779812] pci 0000:02:17.0: BAR 1: assigned [mem 0xfe60f000-0xfe60ffff]

[   85.779835] pci 0000:02:17.0: BAR 0: assigned [io  0x6cc0-0x6cff]

[   85.779951] virtio-pci 0000:02:17.0: enabling device (0000 -> 0003)

[   85.833435] virtio-pci 0000:02:17.0: virtio_pci: leaving for legacy driver

[   85.846894] virtio-pci 0000:02:17.0: irq 61 for MSI/MSI-X

[   85.846927] virtio-pci 0000:02:17.0: irq 62 for MSI/MSI-X

[   86.013107] pci 0000:02:18.0: [1af4:1001] type 00 class 0x010000

[   86.013199] pci 0000:02:18.0: reg 0x10: [io  0x0000-0x003f]

[   86.013241] pci 0000:02:18.0: reg 0x14: [mem 0x00000000-0x00000fff]

[   86.013868] pci 0000:02:18.0: BAR 1: assigned [mem 0xfe610000-0xfe610fff]

[   86.013903] pci 0000:02:18.0: BAR 0: no space for [io  size 0x0040]

[   86.013925] pci 0000:02:18.0: BAR 0: failed to assign [io  size 0x0040]

[   86.014010] virtio-pci 0000:02:18.0: enabling device (0000 -> 0002)

[   86.057575] virtio-pci 0000:02:18.0: virtio_pci: leaving for legacy driver

[   86.088217] virtio-pci: probe of 0000:02:18.0 failed with error -12

We went through the kernel codes which processing the hotplug pci devices, the call stack is:

acpi_hotplug_work_fn

â>enable_slot

->__pci_bus_assign_resources

->pci_bus_alloc_resource

->pci_bus_alloc_from_region

->allocate_resource

->find_resource

->pcibios_align_resource

The failure comes with pcibios_align_resource().

resource_size_t

pcibios_align_resource(void *data, const struct resource *res,

resource_size_t size, resource_size_t align)

{

struct pci_dev *dev = data;

resource_size_t start = res->start;

if (res->flags & IORESOURCE_IO) {

if (skip_isa_ioresource_align(dev))

return start;

if (start & 0x300)

start = (start + 0x3ff) & ~0x3ff; Ãhere.

With the above logic, Only the bellow IO addresses are available for virtio-blk:

[0x6000-0x603f], [0x6040-0x607f], [0x6080-0x60bf], [0x60c0-0x60ff]

[0x6400-0x643f], [0x6440-0x647f], [0x6480-0x64bf], [0x64c0-0x64ff]

[0x6800-0x683f], [0x6840-0x687f], [0x6880-0x68bf], [0x68c0-0x68ff]

[0x6c00-0x6c3f], [0x6c40-0x6c7f], [0x6c80-0x6cbf], [0x6cc0-0x6cff]

So the number is just 16.

We have noticed the comments above pcibios_align_resource():

+/*

+ * We need to avoid collisions with `mirrored' VGA ports

+ * and other strange ISA hardware, so we always want the

+ * addresses to be allocated in the 0x000-0x0ff region

+ * modulo 0x400.

+ *

But we still didnât quite understand about this, does anyone know about this ?

Or could we skip this checking for standard pci devices ?

Thanks,

Hailiang