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

[Alexander Bezzubikov]

2017-07-24 23:46 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.
>
> But this is exactly what this patch does as the property is added to all
> buses and default to 1 (1 extra bus).
>
>> > 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)
>
> You can still plug in PCIe devices there.
>
>> Second (and more important), if we want 2 or more PCI
>> devices we would loose both IO ports space and bus numbers.
>
> What I am saying is maybe default should not be 1.

The only sensible variant left is 0.
But as we want pcie-pci-bridge to be used for every legacy PCI device
on q35 machine, every time one hotplugs the bridge into the root port,
he must be sure rp's prop value >0 (for Linux). I'm not sure
that it is a very convenient way to utilize the bridge - always remember
to set property.
Another way - we can set this to 0 by default, and to 1 for pcie-root-port,
and recommend to use it for hotplugging of the pcie-pci-bridge itself.

>
>> >
>> > >
>> > > 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