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

[Michael S. Tsirkin]

On Wed, Sep 29, 2021 at 04:41:49PM +0300, Marcel Apfelbaum wrote:
> Hi Michael,
> 
> On Mon, Sep 27, 2021 at 12:49 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> 
>     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?
> 
> 
> We need the IO space for the legacy PCI bridges, otherwise an empty PCI bridge
> will become unusable.

Maybe we should go back to using OSC then ... the issue
is we can't then mix acpi and native hotplug for bridges.


> 
> 
>     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?
> 
> 
> I suppose it is possible.
> 
> Thanks,
> Marcel
>  
> 
> 
>     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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>