Re: [Qemu-devel] [RFC PATCH v2 5/6] hw/pci: add bus_reserve property to pcie-root-port

[Marcel Apfelbaum]

On 23/07/2017 15:22, Michael S. Tsirkin wrote:
> On Sun, Jul 23, 2017 at 01:15:42AM +0300, Aleksandr Bezzubikov wrote:
> > To enable hotplugging of a newly created pcie-pci-bridge,
> > we need to tell firmware (SeaBIOS in this case)

Hi Michael,

> Presumably, EFI would need to support this too?

Sure, Eduardo added to CC, but he is in PTO now.

> > to reserve
> > additional buses for pcie-root-port, that allows us to
> > hotplug pcie-pci-bridge into this root port.
> > The number of buses to reserve is provided to the device via a corresponding
> > property, and to the firmware via new PCI capability (next patch).
> > The property's default value is 1 as we want to hotplug at least 1 bridge.
>
> If so you should just teach firmware to allocate one bus #
> unconditionally.

That would be a problem for the PCIe machines, since each PCIe
devices is plugged in a different bus and we are already
limited to 256 PCIe devices. Allocating an extra-bus always
would really limit the PCIe devices we can use.

> But why would that be so? What's wrong with a device
> directly in the root port?

First, plugging a legacy PCI device into a PCIe Root Port
looks strange at least, and it can;t be done on real HW anyway.
(incompatible slots)

Second (and more important), if we want 2 or more PCI
devices we would loose both IO ports space and bus numbers.

> > Signed-off-by: Aleksandr Bezzubikov <address@hidden>
> > ---
> >   hw/pci-bridge/pcie_root_port.c | 1 +
> >   include/hw/pci/pcie_port.h     | 3 +++
> >   2 files changed, 4 insertions(+)
> >
> > diff --git a/hw/pci-bridge/pcie_root_port.c b/hw/pci-bridge/pcie_root_port.c
> > index 4d588cb..b0e49e1 100644
> > --- a/hw/pci-bridge/pcie_root_port.c
> > +++ b/hw/pci-bridge/pcie_root_port.c
> > @@ -137,6 +137,7 @@ static void rp_exit(PCIDevice *d)
> >   static Property rp_props[] = {
> >       DEFINE_PROP_BIT(COMPAT_PROP_PCP, PCIDevice, cap_present,
> >                       QEMU_PCIE_SLTCAP_PCP_BITNR, true),
> > +    DEFINE_PROP_UINT8("bus_reserve", PCIEPort, bus_reserve, 1),
> >       DEFINE_PROP_END_OF_LIST()
> >   };
> >   
> > diff --git a/include/hw/pci/pcie_port.h b/include/hw/pci/pcie_port.h
> > index 1333266..1b2dd1f 100644
> > --- a/include/hw/pci/pcie_port.h
> > +++ b/include/hw/pci/pcie_port.h
> > @@ -34,6 +34,9 @@ struct PCIEPort {
> >   
> >       /* pci express switch port */
> >       uint8_t     port;
> > +
> > +    /* additional buses to reserve on firmware init */
> > +    uint8_t     bus_reserve;
> >   };
> >   
> >   void pcie_port_init_reg(PCIDevice *d);
>
> So here is a property and it does not do anything.
> It makes it easier to work on series maybe, but review
> is harder since we do not see what it does at all.
> Please do not split up patches like this - you can maintain
> it split up in your branch if you like and merge before sending.

Agreed, Alexandr please merge patches 4-5-6 for your next submission.

Thanks,
Marcel


> > --
> > 2.7.4