[SeaBIOS] [PATCH] fw/pci: do not automatically allocate IO region for PCIe bridges

Marcel Apfelbaum marcel at redhat.com
Sun Dec 6 16:29:47 CET 2015

PCIe downstream ports (Root Ports and switches Downstream Ports) appear
to firmware as PCI-PCI bridges and a 4K IO space is allocated for them
even if there is no device behind them requesting IO space,
all that for hotplug purpose.

However, PCIe devices can work without IO, so there is no need
to allocate IO space for hotplug.

Signed-off-by: Marcel Apfelbaum <marcel at redhat.com>
 - This patch fixes a 15 PCIe Root ports limitation when used with
   virtio pci-express devices having no IO space requirements:
      <qemu cmd line> -M q35 `for i in {1..15}; do echo -device ioh3420,chassis=$i,id=b$i -device virtio-net-pci,bus=b$i,disable-legacy=on; done`
 - There is a patch on QEMU devel list that tackles the problem from another angle
   by allowing PCIe downstream ports to not forward IO requests.
 - The mentioned patch is of course not enough and also requires management software intervention.


 src/fw/pciinit.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/fw/pciinit.c b/src/fw/pciinit.c
index 7b8aab7..4b37792 100644
--- a/src/fw/pciinit.c
+++ b/src/fw/pciinit.c
@@ -736,7 +736,9 @@ static int pci_bios_check_devices(struct pci_bus *busses)
             if (pci_region_align(&s->r[type]) > align)
                  align = pci_region_align(&s->r[type]);
             u64 sum = pci_region_sum(&s->r[type]);
-            if (!sum && hotplug_support)
+            int res_opt = (type == PCI_REGION_TYPE_IO) &&
+                          pci_find_capability(s->bus_dev, PCI_CAP_ID_EXP, 0);
+            if (!sum && hotplug_support && !res_opt)
                 sum = align; /* reserve min size for hot-plug */
             u64 size = ALIGN(sum, align);
             int is64 = pci_bios_bridge_region_is64(&s->r[type],

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