Re: [PATCH] pci: Account for virtual buses in pci_acs_path_enabled

[David Ahern <dsahern@gmail.com>]

On 8/8/12 12:00 AM, Bjorn Helgaas wrote:
> On Tue, Aug 7, 2012 at 2:50 PM, Don Dutile <ddutile@redhat.com> wrote:
>> On 08/06/2012 04:47 PM, Bjorn Helgaas wrote:
>>>
>>> On Sun, Aug 5, 2012 at 11:55 PM, Alex Williamson
>>> <alex.williamson@redhat.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@redhat.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? :)
>
> I don't claim that :)  I just wanted to explore other possible
> solutions.  Changing every loop that searches the parent chain so it
> knows about this SR-IOV oddity doesn't seem like the ideal solution,
> though maybe it's the best we can do given the constraints.
>
>>> 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.
>
> I think an SR-IOV device can consume multiple bus numbers even without
> ARI (in fact, I think ARI  reduces the number of bus numbers the
> device requires ... e.g., a PF and 15 VFs would require two bus
> numbers without ARI (04:00.0 - 04:00.7 and 05:00.0 - 05:00.7) but only
> one bus number with ARI (04:00.0 - 04:01.7)).  (I think "04:01.7" is
> how Linux would represent the 8-bit function number ARI gives you.
> You could also think of it as "04:00.0f")
>
>> So, VF devices should be attached to same bus->devices list as it's PF.
>
> I don't think it works that way today, does it?  In the SR-IOV spec
> example in sec 2.1.2:
>
>    PF 0 at 04:00.0
>    ARI Capable is set
>    First VF Offset = 1, VF Stride = 1, NumVFs = 600
>
> I think we have three separate bus->devices lists:
>
>    pci_bus 04: devices list contains PF 0 and VF 0,1 through VF 0,255
>    pci_bus 05: devices list contains VF 0,256 - VF 0,511
>    pci_bus 06: devices list contains VF 0,512 - VF 0,600
>
>> 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.
>
> Maybe every VF *should* have the same dev->bus pointer as the PF, but
> I don't think it does today.  I think we only store the bus number in
> the struct pci_bus, so if we *did* give all the VFs the same dev->bus
> pointer and put all the VFs in the same bus->devices list, we'd have
> to store the bus number elsewhere, e.g., in the struct pci_dev.
>
> That might make sense, but the magnitude of a change like that makes
> my head hurt --  it would affect drivers, arch code, config accessors,
> etc.
>

Perhaps I misunderstand your point. VF's have shown up like this for 
quite a while (e.g., running 3.6.0-rc1):

05:00.0 Ethernet controller: Intel Corporation 82576 Gigabit Network 
Connection (rev 01)
05:00.1 Ethernet controller: Intel Corporation 82576 Gigabit Network 
Connection (rev 01)
06:10.0 Ethernet controller: Illegal Vendor ID Device ffff (rev 01)
06:10.1 Ethernet controller: Illegal Vendor ID Device ffff (rev 01)
..
06:11.5 Ethernet controller: Illegal Vendor ID Device ffff (rev 01)

05:00.{0,1} are the PF's and the 06:* are the VF's (BTW, the 'Illegal 
Vendor ID' is new to 3.6; in 3.5 the VF's show as 'Intel Corporation 
82576 Virtual Function' but that's a topic for a different thread I guess).

David