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

["Singh, Brijesh" <brijesh.singh@amd.com>]

Thanks for adding Alex.

Adding Suravee.


On 3/29/19 11:49 AM, Alex Williamson wrote:
> [Cc +Brijesh]
> 
> Hi Brijesh, will the change below require the IVRS to be updated to
> include aliases for all BDF ranges behind a conventional bridge?  I
> think the Linux code handles this regardless of the firmware provided
> aliases, but is it required per spec for the ACPI tables to include
> bridge aliases?  Thanks,
> 

We do need to includes aliases in ACPI table. We need to populate the
IVHD type 0x43 and 0x4 for alias range start and end. I believe host
IVRS would contain similar information.

Suravee, please correct me if I am missing something?

thanks

> Alex
> 
> On Tue, 26 Mar 2019 16:55:19 -0600
> Alex Williamson <alex.williamson@redhat.com> 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
>>
>> 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 <alex.williamson@redhat.com>
>> ---
>>   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;
>>   }
>>
>