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?
Bjorn
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