Re: [Qemu-ppc] [Qemu-devel] [PATCH] spapr/pci: populate PCI DT in reverse order

[Greg Kurz]

On Sat, 25 Feb 2017 20:39:18 +1100
Alexey Kardashevskiy <address@hidden> wrote:

> On 22/02/17 21:56, Greg Kurz wrote:
> > From: Greg Kurz <address@hidden>
> > 
> > Since commit 1d2d974244c6 "spapr_pci: enumerate and add PCI device tree", 
> > QEMU
> > populates the PCI device tree in the opposite order compared to SLOF.
> > 
> > Before 1d2d974244c6:
> > 
> > Populating /address@hidden
> >                      00 0000 (D) : 1af4 1000    virtio [ net ]
> >                      00 0800 (D) : 1af4 1001    virtio [ block ]
> >                      00 1000 (D) : 1af4 1009    virtio [ network ]
> > Populating /address@hidden/address@hidden
> > 
> > 7e5294b8 :  /address@hidden
> > 7e52b998 :  |-- address@hidden
> > 7e52c0c8 :  |-- address@hidden
> > 7e52c7e8 :  +-- address@hidden ok
> > 
> > Since 1d2d974244c6:
> > 
> > Populating /address@hidden
> >                      00 1000 (D) : 1af4 1009    virtio [ network ]
> > Populating /address@hidden/address@hidden
> >                      00 0800 (D) : 1af4 1001    virtio [ block ]
> >                      00 0000 (D) : 1af4 1000    virtio [ net ]
> > 
> > 7e5e8118 :  /address@hidden
> > 7e5ea6a0 :  |-- address@hidden
> > 7e5eadb8 :  |-- address@hidden
> > 7e5eb4d8 :  +-- address@hidden ok
> > 
> > This behaviour change is not actually a bug since no assumptions should be
> > made on DT ordering. But it has no real justification either, other than
> > being the consequence of the way fdt_add_subnode() inserts new elements
> > to the front of the FDT rather than adding them to the tail.
> > 
> > This patch reverts to the historical SLOF ordering by walking PCI devices
> > in reverse order. This reconciles pseries with x86 machine types behavior.
> > It is expected to make things easier when porting existing applications to
> > power.
> > 
> > Signed-off-by: Greg Kurz <address@hidden>
> > Tested-by: Thomas Huth <address@hidden>
> > Reviewed-by: Nikunj A Dadhania <address@hidden>
> > (slight update to the changelog)
> > Signed-off-by: Greg Kurz <address@hidden>
> > ---
> >  hw/pci/pci.c         |   28 ++++++++++++++++++++++++++++
> >  hw/ppc/spapr_pci.c   |   12 ++++++------
> >  include/hw/pci/pci.h |    4 ++++
> >  3 files changed, 38 insertions(+), 6 deletions(-)
> > 
> > David,
> > 
> > This patch was posted and already discussed during 2.5 development:
> > 
> > http://patchwork.ozlabs.org/patch/549925/
> > 
> > The "consensus" at the time was that guests should not rely on device
> > ordering (i.e. use persistent naming instead).
> > 
> > I got recently contacted by OpenStack people who had several complaints
> > about the reverse ordering of PCI devices in pseries: different behavior
> > between ppc64 and x86, lots of time spent in debugging when porting
> > applications from x86 to ppc64 before realizing that it is caused by the
> > reverse ordering, necessity to carry hacky workarounds...  
> 
> 
> x86 does not have a device tree, and PCI id (bus:slot:fn) is the same
> regardless the scanning order, i.e. "lspci" will show the same picture with
> either order.
> 
> How could OpenStack tell the difference and require workaround for what
> precisely?
> 
> I am definitely missing the point here...
> 

NICs get probed in reverse order and are assigned different names compared
to the same setup on x86 (i.e. eth0 becomes eth1). They end up using wrong
network settings.

The same happens when using Nova libvirt driver which puts each disk on its
own PCI slot (vda becomes vdb).

This is usually avoided with persistent naming but as mentioned below, this
would require a lot of extra work, *just* because ppc64 guests don't do like
everybody else.

> 
> > 
> > One strong argument against handling this properly with persistent naming
> > is that it requires systemd/udev. This option is considered as painful
> > with CirrOS, which aims at remaining as minimal as possible and is widely
> > used in the OpenStack ecosystem.
> > 
> > Would you re-consider your position and apply this patch ?
> > 
> > Cheers.
> > 
> > diff --git a/hw/pci/pci.c b/hw/pci/pci.c
> > index a563555e7da7..273f1e46025a 100644
> > --- a/hw/pci/pci.c
> > +++ b/hw/pci/pci.c
> > @@ -1530,6 +1530,34 @@ static const pci_class_desc pci_class_descriptions[] 
> > =
> >      { 0, NULL}
> >  };
> >  
> > +static void pci_for_each_device_under_bus_reverse(PCIBus *bus,
> > +                                                  void (*fn)(PCIBus *b,
> > +                                                             PCIDevice *d,
> > +                                                             void *opaque),
> > +                                                  void *opaque)
> > +{
> > +    PCIDevice *d;
> > +    int devfn;
> > +
> > +    for (devfn = 0; devfn < ARRAY_SIZE(bus->devices); devfn++) {
> > +        d = bus->devices[ARRAY_SIZE(bus->devices) - 1 - devfn];
> > +        if (d) {
> > +            fn(bus, d, opaque);
> > +        }
> > +    }
> > +}
> > +
> > +void pci_for_each_device_reverse(PCIBus *bus, int bus_num,
> > +                         void (*fn)(PCIBus *b, PCIDevice *d, void *opaque),
> > +                         void *opaque)
> > +{
> > +    bus = pci_find_bus_nr(bus, bus_num);
> > +
> > +    if (bus) {
> > +        pci_for_each_device_under_bus_reverse(bus, fn, opaque);
> > +    }
> > +}
> > +
> >  static void pci_for_each_device_under_bus(PCIBus *bus,
> >                                            void (*fn)(PCIBus *b, PCIDevice 
> > *d,
> >                                                       void *opaque),
> > diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
> > index fd6fc1d95344..2a20c2a140fc 100644
> > --- a/hw/ppc/spapr_pci.c
> > +++ b/hw/ppc/spapr_pci.c
> > @@ -1782,9 +1782,9 @@ static void spapr_populate_pci_devices_dt(PCIBus 
> > *bus, PCIDevice *pdev,
> >      s_fdt.fdt = p->fdt;
> >      s_fdt.node_off = offset;
> >      s_fdt.sphb = p->sphb;
> > -    pci_for_each_device(sec_bus, pci_bus_num(sec_bus),
> > -                        spapr_populate_pci_devices_dt,
> > -                        &s_fdt);
> > +    pci_for_each_device_reverse(sec_bus, pci_bus_num(sec_bus),
> > +                                spapr_populate_pci_devices_dt,
> > +                                &s_fdt);
> >  }
> >  
> >  static void spapr_phb_pci_enumerate_bridge(PCIBus *bus, PCIDevice *pdev,
> > @@ -1953,9 +1953,9 @@ int spapr_populate_pci_dt(sPAPRPHBState *phb,
> >      s_fdt.fdt = fdt;
> >      s_fdt.node_off = bus_off;
> >      s_fdt.sphb = phb;
> > -    pci_for_each_device(bus, pci_bus_num(bus),
> > -                        spapr_populate_pci_devices_dt,
> > -                        &s_fdt);
> > +    pci_for_each_device_reverse(bus, pci_bus_num(bus),
> > +                                spapr_populate_pci_devices_dt,
> > +                                &s_fdt);
> >  
> >      ret = spapr_drc_populate_dt(fdt, bus_off, OBJECT(phb),
> >                                  SPAPR_DR_CONNECTOR_TYPE_PCI);
> > diff --git a/include/hw/pci/pci.h b/include/hw/pci/pci.h
> > index 6983f13745a5..9349acbfb278 100644
> > --- a/include/hw/pci/pci.h
> > +++ b/include/hw/pci/pci.h
> > @@ -429,6 +429,10 @@ int pci_bus_numa_node(PCIBus *bus);
> >  void pci_for_each_device(PCIBus *bus, int bus_num,
> >                           void (*fn)(PCIBus *bus, PCIDevice *d, void 
> > *opaque),
> >                           void *opaque);
> > +void pci_for_each_device_reverse(PCIBus *bus, int bus_num,
> > +                                 void (*fn)(PCIBus *bus, PCIDevice *d,
> > +                                            void *opaque),
> > +                                 void *opaque);
> >  void pci_for_each_bus_depth_first(PCIBus *bus,
> >                                    void *(*begin)(PCIBus *bus, void 
> > *parent_state),
> >                                    void (*end)(PCIBus *bus, void *state),
> > 
> >   
> 
>

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