Re: [PULL 1/5] hw/pcie-root-port: Fix hotplug for PCI devices requiring IO

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Aug 03, 2021 at 04:52:03PM -0400, Michael S. Tsirkin wrote:
> From: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> 
> Q35 has now ACPI hotplug enabled by default for PCI(e) devices.
> As opposed to native PCIe hotplug, guests like Fedora 34
> will not assign IO range to pcie-root-ports not supporting
> native hotplug, resulting into a regression.
> 
> Reproduce by:
>     qemu-bin -M q35 -device pcie-root-port,id=p1 -monitor stdio
>     device_add e1000,bus=p1
> In the Guest OS the respective pcie-root-port will have the IO range
> disabled.
> 
> Fix it by setting the "reserve-io" hint capability of the
> pcie-root-ports so the firmware will allocate the IO range instead.
> 
> Acked-by: Igor Mammedov <imammedo@redhat.com>
> Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
> Message-Id: <20210802090057.1709775-1-marcel@redhat.com>
> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  hw/pci-bridge/gen_pcie_root_port.c | 5 +++++
>  1 file changed, 5 insertions(+)

This change, when combined with the switch to ACPI based hotplug by
default, is responsible for a significant regression in QEMU 6.1.0

It is no longer possible to have more than 15 pcie-root-port devices
added to a q35 VM in 6.1.0.  Before this I've had as many as 80+ devices
present before I stopped trying to add more.

  https://gitlab.com/qemu-project/qemu/-/issues/641

This regression is significant, because it has broken the out of the
box default configuration that OpenStack uses for booting all VMs.
They add 16 pcie-root-ports by defalt to allow empty slots for device
hotplug under q35 [1].

> diff --git a/hw/pci-bridge/gen_pcie_root_port.c b/hw/pci-bridge/gen_pcie_root_port.c
> index ec9907917e..20099a8ae3 100644
> --- a/hw/pci-bridge/gen_pcie_root_port.c
> +++ b/hw/pci-bridge/gen_pcie_root_port.c
> @@ -28,6 +28,7 @@ OBJECT_DECLARE_SIMPLE_TYPE(GenPCIERootPort, GEN_PCIE_ROOT_PORT)
>          (GEN_PCIE_ROOT_PORT_AER_OFFSET + PCI_ERR_SIZEOF)
>  
>  #define GEN_PCIE_ROOT_PORT_MSIX_NR_VECTOR       1
> +#define GEN_PCIE_ROOT_DEFAULT_IO_RANGE          4096
>  
>  struct GenPCIERootPort {
>      /*< private >*/
> @@ -75,6 +76,7 @@ static bool gen_rp_test_migrate_msix(void *opaque, int version_id)
>  static void gen_rp_realize(DeviceState *dev, Error **errp)
>  {
>      PCIDevice *d = PCI_DEVICE(dev);
> +    PCIESlot *s = PCIE_SLOT(d);
>      GenPCIERootPort *grp = GEN_PCIE_ROOT_PORT(d);
>      PCIERootPortClass *rpc = PCIE_ROOT_PORT_GET_CLASS(d);
>      Error *local_err = NULL;
> @@ -85,6 +87,9 @@ static void gen_rp_realize(DeviceState *dev, Error **errp)
>          return;
>      }
>  
> +    if (grp->res_reserve.io == -1 && s->hotplug && !s->native_hotplug) {
> +        grp->res_reserve.io = GEN_PCIE_ROOT_DEFAULT_IO_RANGE;
> +    }
>      int rc = pci_bridge_qemu_reserve_cap_init(d, 0,
>                                                grp->res_reserve, errp);
>  
> -- 
> MST
> 
> 

Regards,
Daniel

[1] https://github.com/openstack/tripleo-heat-templates/blob/7a6cd0640ec390a330f5699d8ed60f71b2a9f514/deployment/nova/nova-compute-container-puppet.yaml#L462-L472
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