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

[Denis V. Lunev]

On 11/4/22 15:11, Daniel P. Berrangé wrote:
> 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.
Thanks for a note!