Re: [PATCH] qemu: add pcie-to-pci-bridge for q35 machines

On Fri, Nov 04, 2022 at 03:01:38PM +0100, Denis V. Lunev wrote: > On 11/4/22 14:41, Daniel P. Berrangé wrote: >> On Fri, Nov 04, 2022 at 01:03:52PM +0100, Peter Krempa wrote: >>> On Fri, Nov 04, 2022 at 16:43:00 +0600, Oleg Vasilev wrote: >>>> Hotplugging PCI devices on pc-i440fx machines is supported without >>>> additional configuration. On q35, pcie-to-pci-bridge needs to be added >>>> prior to the machine startup in order to support hotplug [1]. >>>> >>>> The idea is to support the same workflow for creating VMs in q35, as was >>>> in pc. Namely, do not require additional configuration when hotplugging >>>> is needed. Otherwise, all libvirt clients need to be updated which there >>>> are a lot and they are maintained by different parties. >>>> >>>> Instead, a pcie-to-pci-bridge better be created by default, so that PCI >>>> slots are readily available. Might be a good idea to make it configurable >>>> in the future. >> The goal of q35 is to provide a PCI-e topology though, and we want >> devices to be exposed as PCI-e endpoints, with no legacy PCI usage >> by default. >> >> When a guest XML is provided for cold boot, libvirt will ensure >> that all devices are placed onto the PCI-e bus via pcie-root-ports >> >> With this proposal hotplugging a device will cause it to be placed >> on a PCI bus instead, if there are no spare pcie-root-port available. >> >> This behavioural difference between cold plug and hot plug is very >> undesirable IMHO, as it is counter to our goal of avoiding legacy >> PCI on PCI-e machines. > that is an interesting point which I have initially missed. I have > checked our investigation log and found that we have > potentially made a mistake with making compatibility > check with e1000 device instead of virtio, which is > our default. This could be the reason. virtio devices are particularly sensitive to PCI vs PCI-e placement, because if placed on PCI, they get setup in transitional-mode, where they advertize the legacy PCI IDs and virtio 0.9/1.0 is negotiated, while if placed on PCI-E, they get unconditionally setup in modern mode with the modern PCI IDs and virtio 1.0 only.