Re: [libvirt] How should libvirt apps enable virtio-pci for aarch64?

Hello! Since we are discussing this, i have a question. Why do we construct exactly this thing? What is "dmi-to-pci-bridge" and why do we need it? I guess this is something PC-specific, may be this layout has been copied from some real PC model, but i don't see any practical sense in it. Also, there are two problems with "pci-bridge": 1. Live migration of this thing in qemu is broken. After migration the bridge screws up, lspci says "invalid header", and i don't know whether it actually works because i never attach anything behind it, because of (2). 2. After pcie-root we have PCI-X, which supports MSI-X. And after pci-bridge we seem to have a plain PCI, which supports only plain MSI. The problem here is that virtio seems to work only with MSI-X in any advanced mode (multiqueue, vhost, etc). If i place it behind the bridge (and default libvirt's logic is to place the device there), MSI-X will not work. The same applies to passthrough VFIO devices. This is especially painful because on real-life ARM64 platforms builtin hardware seems to mandate MSI-X. For example on ThunderX NIC driver simply does not support anything except MSI-X. I like this one, and IMHO this would be nice to have regardless of the default. Manual assignment of PCI layout is a tedious process, which is not always necessary. I think it is quite logical to allow the user just to say: "I want this device on the PCI bus", and do the rest for him. Also, this would allow to have a simple "Address type" switch in e.g. virt-manager, so that i would be able to easily choose a way which the device uses to connect to the system. Kind regards, Pavel Fedin Expert Engineer Samsung Electronics Research center Russia