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

[Michael S. Tsirkin]

On Wed, Mar 27, 2019 at 05:43:45PM +0100, Auger Eric wrote:
> Hi,
> 
> On 3/27/19 4:32 PM, Michael S. Tsirkin wrote:
> > On Tue, Mar 26, 2019 at 04:55:19PM -0600, Alex Williamson wrote:
> >> 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
> > 
> > 
> > This will break extended address space access though, won't it?
> > things like attempts to take PCI Express badnwidth into
> > consideration by drivers for E2E credit math will also
> > not work ...
> > 
> >>
> >> While the utility of this hack may be limited, this AddressSpace
> >> aliasing is the correct behavior for QEMU to emulate bare metal.
> > 
> > 
> > This one's a valid point.
> > 
> >> 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.
> > 
> > Could you clarify this part a bit pls?
> > 
> >>  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);
> > 
> > Why 0,0?
> 
> The spec says: "If the bridge generates a new Requester ID for a
> transaction forwarded from the secondary interface to the primary
> interface, the bridge assigns the PCI Express Requester ID using its
> secondary interface’s Bus Number and sets both the Device Number and
> Function Number fields to zero.". I guess it is the reason?

Ah.

> > 
> >> +                bus = iommu_bus;
> >> +            } else {
> >> +                devfn = parent->devfn;
> >> +                bus = parent_bus;
> Alex, please could you clarify this case as well , comment?
> 
> Thanks
> 
> Eric


right.

> >> +            }
> >> +        }
> >> +
> >> +        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;
> >>  }