Date:	Tue, 07 Aug 2012 17:50:46 -0400
From:	Don Dutile <ddutile@...hat.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
CC:	Alex Williamson <alex.williamson@...hat.com>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	dsahern@...il.com
Subject: Re: [PATCH] pci: Account for virtual buses in pci_acs_path_enabled

On 08/06/2012 04:47 PM, Bjorn Helgaas wrote:
> On Sun, Aug 5, 2012 at 11:55 PM, Alex Williamson
> <alex.williamson@...hat.com>  wrote:
>> On Sun, 2012-08-05 at 23:30 -0600, Bjorn Helgaas wrote:
>>> On Sat, Aug 4, 2012 at 12:19 PM, Alex Williamson
>>> <alex.williamson@...hat.com>  wrote:
>>>> It's possible to have buses without an associated bridge
>>>> (bus->self == NULL).  SR-IOV can generate such buses.  When
>>>> we find these, skip to the parent bus to look for the next
>>>> ACS test.
>>>
>>> To make sure I understand the problem here, I think you're referring
>>> to the situation where an SR-IOV device can span several bus numbers,
>>> e.g., the "VFs Spanning Multiple Bus Numbers" implementation note in
>>> the SR-IOV 1.1 spec, sec. 2.1.2.
>>>
>>> It says "All PFs must be located on the Device's captured Bus Number"
>>> -- I think that means every PF will be directly on a bridge's
>>> secondary bus and hence will have a valid dev->bus->self pointer.
>>>
>>> However, VFs need not be on the same bus number.  If a VF is on
>>> (captured Bus Number plus 1), I think we allocate a new struct pci_bus
>>> for it, but there's no P2P bridge that leads to that bus, so the
>>> bus->self pointer is probably NULL.
>>
>> Yes, exactly.  virtfn_add_bus() is where we're creating this new bus.
>>
>>> This makes me quite nervous, because I bet there are many places that
>>> assume every non-root bus has a valid bus->self pointer  -- I know I
>>> certainly had that assumption.
>>>
>>> I looked at callers of pci_is_root_bus(), and at first glance, it seems like
>>> iommu_init_device(), intel_iommu_add_device(), pci_acs_path_enabled(),
>>
>>
>> These 3 are handled by this patch, plus the intel and amd iommu patches
>> I sent.
>>
>>> pci_get_interrupt_pin(), pci_common_swizzle(),
>>
>> If sr-iov is the only source of these virtual buses, these are probably
>> ok since VFs don't support INTx.
>>
>>> pci_find_upstream_pcie_bridge(), and
>>
>> Here the pci_is_root_bus() is after a pci_is_pcie() check, so again if
>> sr-iov only (and assuming VFs properly report PCIe capability), we
>> shouldn't stumble on it.
>>
>>> pci_bus_release_bridge_resources() all might have similar problems.
>>
>> This one might deserve further investigation.  Thanks,
>
> We can fix all these places piecemeal, but that doesn't feel like a
> very satisfying solution.  It makes it much harder to know that each
> place is correct, and this oddity of a bus with no upstream bridge is
> still lying around, waiting to bite us again later.
>
> What other possible ways of fixing this do we have?  Could we set
> bus->self (multiple buses would then point to the same bridge, and I
> don't know if that would break something)?  Add something like a
> pci_upstream_p2p_bridge() interface that would encapsulate traversing
   ^^^ and this name will reduce the confusion? :)

> the bus->parent and bus->self links?
>
> Since these fake VF buses don't have a bridge that points to them, I
Well, they aren't fake busses, just ARI-identifiers, which translate the B:D.F/8:5.3
format to simply a 16-bit i.d.
So, VF devices should be attached to same bus->devices list as it's PF.
pci_dev->bus should be same bus ptr as PF's pci_dev as well, since the
VF uses all that's busses resources, support functions (cfg, dma-ops, etc.) as well.
Searching the driver/pci area, support of functions like AER want the
bus struct that's receiving/handling the PCIe error, associated (hw) port, etc.,
so another reason the VF's pci-dev bus ptr should be the same as the PF's.
Logically, ARI-based VFs with a 'bus-num' value != PF bus-num value make
a point-to-point PCIe link look more like a parallel-bus with a different
identifier parsing -- diff. interpretation of a 16-bit field.

> think the only place we keep a pointer to them is in the parent bus's
> "children" list (updated in pci_add_new_bus()).  And now I'm confused
> about when we should use bus->children and when we should use
> bus->devices and why we should have both.
well, children are child busses; devices are all devices, bus-bridge & endpt devices.
As for use.... seems like children should be traversed when doing bus ops.

>
> Does pci_walk_bus() work correctly with these VFs on fake buses?  It
> doesn't use "children", so I can't see how it would ever find them.
>
as I read pci_walk_bus(), it won't work for VFs attached to a bus-num-id
that doesn't match the PF's bus-num.
sure glad the VFs don't use/need pci_walk_bus()! :o !
Seems like a bug in that algorithm....

> Aren't you sorry you opened this can of worms?  :)
>
yeah, aw has a tendency to step in it (worms would be too clean an analogy for Alex!).


>>>> Signed-off-by: Alex Williamson<alex.williamson@...hat.com>
>>>> ---
>>>>
>>>> David Ahern reported an oops from iommu drivers passing NULL into
>>>> this function for the same mistake.  Harden this function against
>>>> assuming bus->self is valid as well.  David, please include this
>>>> patch as well as the iommu patches in your testing.
>>>>
>>>>   drivers/pci/pci.c |   22 +++++++++++++++++-----
>>>>   1 file changed, 17 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index f3ea977..e11a49c 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -2486,18 +2486,30 @@ bool pci_acs_enabled(struct pci_dev *pdev, u16 acs_flags)
>>>>   bool pci_acs_path_enabled(struct pci_dev *start,
>>>>                            struct pci_dev *end, u16 acs_flags)
>>>>   {
>>>> -       struct pci_dev *pdev, *parent = start;
>>>> +       struct pci_dev *pdev = start;
>>>> +       struct pci_bus *bus;
>>>>
>>>>          do {
>>>> -               pdev = parent;
>>>> -
>>>>                  if (!pci_acs_enabled(pdev, acs_flags))
>>>>                          return false;
>>>>
>>>> -               if (pci_is_root_bus(pdev->bus))
>>>> +               bus = pdev->bus;
>>>> +
>>>> +               if (pci_is_root_bus(bus))
>>>>                          return (end == NULL);
>>>>
>>>> -               parent = pdev->bus->self;
>>>> +               /*
>>>> +                * Skip buses without an associated bridge.  In this
>>>> +                * case move to the parent and continue.
>>>> +                */
>>>> +               while (!bus->self) {
>>>> +                       if (!pci_is_root_bus(bus))
>>>> +                               bus = bus->parent;
>>>> +                       else
>>>> +                               return (end == NULL);
>>>> +               }
>>>> +
>>>> +               pdev = bus->self;
>>>>          } while (pdev != end);
>>>>
>>>>          return true;
>>>>
>>
>>
>>
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