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

[Daniel P. Berrangé]

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.

With regards,
Daniel
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