On Tue, Mar 15, 2016 at 07:48:17PM -0500, Bjorn Helgaas wrote:
> On Mon, Mar 14, 2016 at 10:43:40PM +0000, David Woodhouse wrote:
> > On Thu, 2016-02-25 at 08:38 -0600, Bjorn Helgaas wrote:
> > >
> > > > /*
> > > > - * Look for aliases to or from the given device for exisiting groups. The
> > > > - * dma_alias_devfn only supports aliases on the same bus, therefore the search
> > > > + * Look for aliases to or from the given device for existing groups. DMA
> > > > + * aliases are only supported on the same bus, therefore the search
> > >
> > > I'm trying to reconcile this statement that "DMA aliases are only
> > > supported on the same bus" (which was there even before this patch)
> > > with the fact that pci_for_each_dma_alias() does not have that
> > > limitation.
> >
> > Doesn't it? You can still only set a DMA alias on the same bus with
> > pci_add_dma_alias(), can't you?
>
> I guess it's true that PCI_DEV_FLAGS_DMA_ALIAS_DEVFN and the proposed
> pci_add_dma_alias() only add aliases on the same bus. I was thinking
> about a scenario like this:
>
> 00:00.0 PCIe-to-PCI bridge to [bus 01]
> 01:01.0 conventional PCI device
>
> where I think 01:00.0 is a DMA alias for 01:01.0 because the bridge
> takes ownership of DMA transactions from 01:01.0 and assigns a
> Requester ID of 01:00.0 (secondary bus number, device 0, function 0).
>
> > > > * space is quite small (especially since we're really only looking at pcie
> > > > * device, and therefore only expect multiple slots on the root complex or
> > > > * downstream switch ports). It's conceivable though that a pair of
> > > > @@ -686,11 +692,8 @@ static struct iommu_group *get_pci_alias_group(struct pci_dev *pdev,
> > > > continue;
> > > >
> > > > /* We alias them or they alias us */
> > > > - if (((pdev->dev_flags & PCI_DEV_FLAGS_DMA_ALIAS_DEVFN) &&
> > > > - pdev->dma_alias_devfn == tmp->devfn) ||
> > > > - ((tmp->dev_flags & PCI_DEV_FLAGS_DMA_ALIAS_DEVFN) &&
> > > > - tmp->dma_alias_devfn == pdev->devfn)) {
> > > > -
> > > > + if (dma_alias_is_enabled(pdev, tmp->devfn) ||
> > > > + dma_alias_is_enabled(tmp, pdev->devfn)) {
> > > > group = get_pci_alias_group(tmp, devfns);
> > >
> > > We basically have this:
> > >
> > > for_each_pci_dev(tmp) {
> > > if ()
> > > group = get_pci_alias_group();
> > > ...
> > > }
> >
> > Strictly, that's:
> >
> > for_each_pci_dev(tmp) {
> > if (pdev is an alias of tmp || tmp is an alias of pdev)
> > group = get_pci_alias_group();
> > ...
> > }
>
> OK.
>
> > > I'm trying to figure out why we don't do something like the following
> > > instead:
> > >
> > > callback(struct pci_dev *pdev, u16 alias, void *opaque)
> > > {
> > > struct iommu_group *group;
> > >
> > > group = get_pci_alias_group();
> > > if (group)
> > > return group;
> > >
> > > return 0;
> > > }
> > >
> > > pci_for_each_dma_alias(pdev, callback, ...);
> >
> > And this would be equivalent to
> >
> > for_each_pci_dev(tmp) {
> > if (tmp is an alias of pdev)
> > group = get_pci_alias_group();
> > ...
> > }
> >
> > The "is an alias of" property is not commutative. Perhaps it should be.
> > But that's hard because in some cases the alias doesn't even *exist* as
> > a real PCI device. It's just that you appear to get DMA transactions
> > from a given source-id.
>
> Right. In my example above, 01:00.0 is not a PCI device; it's only a
> Requester ID that is fabricated by the bridge when it forwards DMA
> transactions upstream.
>
> I think I'm confused because I don't really understand IOMMU groups.
>
> Let me explain what I think they are and you can correct me when I go
> wrong. The iommu_group_alloc() comment says "The IOMMU group
> represents the minimum granularity of the IOMMU." So I suppose the
> IOMMU cannot distinguish between devices in a group. All the devices
> in the group use the same set of DMA mappings. Granting device A DMA
> access to a buffer grants the same access to all other members of A's
> IOMMU group.
>
> That would mean my question was fundamentally backwards. In
> get_pci_alias_group(A), we're not trying to figure out what all the
> aliases of A are, which is what pci_for_each_dma_alias() does.
>
> Instead, we're trying to figure out which IOMMU group A belongs to.
> But I still don't quite understand how aliases fit into this. Let's
> go back to my example and assume we've already put 00:00.0 and 01:01.0
> in IOMMU groups:
>
> 00:00.0 PCIe-to-PCI bridge to [bus 01] # in IOMMU group G0
> 01:01.0 conventional PCI device # in IOMMU group G1
>
> I assume these devices are in different IOMMU groups because if the
> bridge generated its own DMA, it would use Requester ID 00:00.0, which
> is distinct from the 01:00.0 it would use when forwarding DMAs from
> its secondary side.
>
> What happens when we add 01:02.0? I think 01:01.0 and 01:02.0 should
> both end up in IOMMU group G1 because the IOMMU will see only
> Requester ID 01:00.0, so it can't distinguish them.
>
> When we add 01:02.0, the ops->add_device() ... ops->device_group()
> path calls pci_device_group(01:02.0):
>
> pci_device_group(01:02.0)
> pci_for_each_dma_alias(01:02.0, get_pci_alias_or_group)
> get_pci_alias_or_group(01:02.0, 01:02.0) # callback
> return 0 # 01:02.0 group not set yet
> get_pci_alias_or_group(00:00.0, 01:00.0) # callback
> return 1 # 00:00.0 is in G0
>
> It seems like we'll assign 01:02.0 to group G0, when I think it should
> be in G1. Where did I go wrong?
Ping?
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