[Qemu-devel] [RFC PATCH] pci: Use PCI aliases when determining device IOMMU address space

[Alex Williamson]

Conventional PCI buses pre-date requester IDs.  An IOMMU cannot
distinguish by devfn & bus between devices in a conventional PCI
topology and therefore we cannot assign them separate AddressSpaces.
By taking this requester ID aliasing into account, QEMU better matches
the bare metal behavior and restrictions, and enables shared
AddressSpace configurations that are otherwise not possible with
guest IOMMU support.

For the latter case, given any example where an IOMMU group on the
host includes multiple devices:

  $ ls  /sys/kernel/iommu_groups/1/devices/
  0000:00:01.0  0000:01:00.0  0000:01:00.1

If we incorporate a vIOMMU into the VM configuration, we're restricted
that we can only assign one of the endpoints to the guest because a
second endpoint will attempt to use a different AddressSpace.  VFIO
only supports IOMMU group level granularity at the container level,
preventing this second endpoint from being assigned:

qemu-system-x86_64 -machine q35... \
  -device intel-iommu,intremap=on \
  -device pcie-root-port,addr=1e.0,id=pcie.1 \
  -device vfio-pci,host=1:00.0,bus=pcie.1,addr=0.0,multifunction=on \
  -device vfio-pci,host=1:00.1,bus=pcie.1,addr=0.1

qemu-system-x86_64: -device vfio-pci,host=1:00.1,bus=pcie.1,addr=0.1: vfio \
0000:01:00.1: group 1 used in multiple address spaces

However, when QEMU incorporates proper aliasing, we can make use of a
PCIe-to-PCI bridge to mask the requester ID, resulting in a hack that
provides the downstream devices with the same AddressSpace, ex:

qemu-system-x86_64 -machine q35... \
  -device intel-iommu,intremap=on \
  -device pcie-pci-bridge,addr=1e.0,id=pci.1 \
  -device vfio-pci,host=1:00.0,bus=pci.1,addr=1.0,multifunction=on \
  -device vfio-pci,host=1:00.1,bus=pci.1,addr=1.1

While the utility of this hack may be limited, this AddressSpace
aliasing is the correct behavior for QEMU to emulate bare metal.

Signed-off-by: Alex Williamson <address@hidden>
---
 hw/pci/pci.c |   33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/hw/pci/pci.c b/hw/pci/pci.c
index 35451c1e9987..38467e676f1f 100644
--- a/hw/pci/pci.c
+++ b/hw/pci/pci.c
@@ -2594,12 +2594,41 @@ AddressSpace *pci_device_iommu_address_space(PCIDevice 
*dev)
 {
     PCIBus *bus = pci_get_bus(dev);
     PCIBus *iommu_bus = bus;
+    uint8_t devfn = dev->devfn;
 
     while(iommu_bus && !iommu_bus->iommu_fn && iommu_bus->parent_dev) {
-        iommu_bus = pci_get_bus(iommu_bus->parent_dev);
+        PCIBus *parent_bus = pci_get_bus(iommu_bus->parent_dev);
+
+        /*
+         * Determine which requester ID alias should be used for the device
+         * based on the PCI topology.  There are no requester IDs on 
convetional
+         * PCI buses, therefore we push the alias up to the parent on each non-
+         * express bus.  Which alias we use depends on whether this is a legacy
+         * PCI bridge or PCIe-to-PCI/X bridge as in chapter 2.3 of the PCIe-to-
+         * PCI bridge spec.  Note that we cannot use pci_requester_id() here
+         * because the resulting BDF depends on the secondary bridge register
+         * programming.  We also cannot lookup the PCIBus from the bus number
+         * at this point for the iommu_fn.  Also, requester_id_cache is the
+         * alias to the root bus, which is usually, but not necessarily always
+         * where we'll find our iommu_fn.
+         */
+        if (!pci_bus_is_express(iommu_bus)) {
+            PCIDevice *parent = iommu_bus->parent_dev;
+
+            if (pci_is_express(parent) &&
+                pcie_cap_get_type(parent) == PCI_EXP_TYPE_PCI_BRIDGE) {
+                devfn = PCI_DEVFN(0, 0);
+                bus = iommu_bus;
+            } else {
+                devfn = parent->devfn;
+                bus = parent_bus;
+            }
+        }
+
+        iommu_bus = parent_bus;
     }
     if (iommu_bus && iommu_bus->iommu_fn) {
-        return iommu_bus->iommu_fn(bus, iommu_bus->iommu_opaque, dev->devfn);
+        return iommu_bus->iommu_fn(bus, iommu_bus->iommu_opaque, devfn);
     }
     return &address_space_memory;
 }