[libvirt-users] pcie-expander-bus doesn't support pcie-pci-bridge and pcie-switch-upstream-port

[Laine Stump]

On 10/17/2018 08:56 AM, Andrea Bolognani wrote:
> On Wed, 2018-10-17 at 10:50 +0800, Han Han wrote:
>> In libvirt, I found pcie-expander-bus controller doesn't support pcie-to-pci-bridge and pcie-switch-upstream-port.
> [...]
>> # virsh -k0 -K0 define /tmp/c.xml
> Aside: the -k and -K virsh options are documented as
>
>   -k | --keepalive-interval=NUM
>                       keepalive interval in seconds, 0 for disable
>   -K | --keepalive-count=NUM
>                       number of possible missed keepalive messages
>
> So -k0 disables keepalive entirely, making -K0 unnecessary :)
>
>> error: Failed to define domain from /tmp/c.xml
>> error: XML error: The device at PCI address 0000:01:00.0 cannot be plugged into the PCI controller with index='1'. It requires a controller that accepts a pcie-to-pci-bridge.
> [...]
>> In function virDomainPCIAddressBusSetModel, I find pcie-expander-bus only supports pcie-root-port and dmi-to-pci-bridge
>>    353     case VIR_DOMAIN_CONTROLLER_MODEL_PCIE_EXPANDER_BUS:
>>    354     ┆   /* 32 slots, no hotplug, only accepts pcie-root-port or
>>    355     ┆   ┆* dmi-to-pci-bridge
>>    356     ┆   ┆*/
>>    357     ┆   bus->flags = (VIR_PCI_CONNECT_TYPE_PCIE_ROOT_PORT |
>>    358     ┆   ┆   ┆   ┆   ┆ VIR_PCI_CONNECT_TYPE_DMI_TO_PCI_BRIDGE);
>>    359     ┆   bus->minSlot = 0;
>>    360     ┆   bus->maxSlot = VIR_PCI_ADDRESS_SLOT_LAST;
>>    361     ┆   break;
> The above is consistent with pcie-root itself, which also doesn't
> support plugging a pcie-to-pci-bridge directly into it but requires
> a pcie-root <- pcie-root-port <- pcie-to-pci-bridge topology.
>
> Now, I don't quite recall *why* that is the case - perhaps Laine
> does? - but I'm sure we had very compelling reasons O:-)

Umm, because Marcel told us that was the proper behavior ? Right, Marcel?


>
>> But it works in qemu:
>> # /usr/libexec/qemu-kvm -machine q35 -m 1024 -device pxb-pcie,bus_nr=250,id=pci.1,bus=pcie.0,addr=0x4 -device pcie-pci-bridge,id=pci.250,bus=pci.1,addr=0x0 -device pcie-pci-bridge,id=pci.251,bus=pci.1,addr=0x1 -spice port=5902,addr=0.0.0.0,disable-ticketing /var/lib/libvirt/images/q35.qcow2
> It should be noted that QEMU in general allows users to make pretty
> unwise device placement choices without so much of a warning, so I
> would not take the above as proof the pcie-to-pci-bridge should be
> allowed to plug into pcie-expander-bus (or pcie-root) directly :)
>