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

[Michael S. Tsirkin]

On Tue, Jul 25, 2017 at 04:50:49PM +0300, Alexander Bezzubikov wrote:
> 2017-07-25 16:43 GMT+03:00 Michael S. Tsirkin <address@hidden>:
> > On Sun, Jul 23, 2017 at 05:13:11PM +0300, Marcel Apfelbaum wrote:
> >> 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.
> >
> > One of the declared advantages of PCIe is easy support for multiple roots.
> > We really should look at that IMHO so we do not need to pile up hacks.
> >
> >> > But why would that be so? What's wrong with a device
> >> > directly in the root port?
> >> >
> >
> > To clarify, my point is we might be wasting bus numbers by reservation
> > since someone might just want to put pcie devices there.
> 
> I think, changing default value to 0 can help us avoid this,
> as no bus reservation by default. If one's surely wants
> to hotplug pcie-pci-bridge into this root port in future,
> the property gives him such an opportunity.
> So, sure need pcie-pci-bridge hotplug -> creating a root port with
> bus_reserve > 0. Otherwise (and default) - just as now, no changes
> in bus topology.

I guess 0 should mean "do not reserve any buses".  So I think we also
need a flag to just avoid the capability altogether.  Maybe -1?  *That*
should be the default.

> >
> >> 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
> 
> 
> 
> -- 
> Alexander Bezzubikov