Re: [PATCH] pci: Account for virtual buses in pci_acs_path_enabled
From: Don Dutile
Date: Tue Aug 07 2012 - 17:50:43 EST
On 08/06/2012 04:47 PM, Bjorn Helgaas wrote:
On Sun, Aug 5, 2012 at 11:55 PM, Alex Williamson
<alex.williamson@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
---
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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