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

[Michael S. Tsirkin]

On Mon, Sep 27, 2021 at 10:33:42AM +0100, Daniel P. Berrangé wrote:
> 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].


Indeed, oops. Thanks for the report!

Going back and looking at seabios code, didn't we get confused?
Shouldn't we have reserved memory and not IO?

I see:
            int resource_optional = pcie_cap && (type == PCI_REGION_TYPE_IO);
            if (!sum && hotplug_support && !resource_optional)
                sum = align; /* reserve min size for hot-plug */


generally maybe we should just add an ACPI-hotplug capability and
teach seabios about it?

Marcel?

> > 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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