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

On Wed, Oct 17, 2018 at 8:56 PM Andrea Bolognani <abologna redhat com> 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 :)

Thanks for your advice.
> 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
>    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:-)

> 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=,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 :)

If  pcie<-pcie-expander-bus<-(pcie-to-pci-bridge | pcie-switch-upstream-port) is an unwise device placement, we'd better to add the reason to code comments or commit msg or documents.
Andrea Bolognani / Red Hat / Virtualization

Best regards,
Han Han
Quality Engineer

Email: hhan redhat com
Phone: +861065339333

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