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SubjectRe: [PATCH] pci: Account for virtual buses in pci_acs_path_enabled
On Tue, Aug 7, 2012 at 2:50 PM, Don Dutile <> wrote:
> On 08/06/2012 04:47 PM, Bjorn Helgaas wrote:
>> On Sun, Aug 5, 2012 at 11:55 PM, Alex Williamson
>> <> wrote:
>>> On Sun, 2012-08-05 at 23:30 -0600, Bjorn Helgaas wrote:
>>>> On Sat, Aug 4, 2012 at 12:19 PM, Alex Williamson
>>>> <> 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,

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