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SubjectRe: [PATCH 1/2] iommu: Add iommu_group_get/set_domain()
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On 2020-06-30 02:03, Lu Baolu wrote:
> Hi Robin,
>
> On 6/29/20 7:56 PM, Robin Murphy wrote:
>> On 2020-06-27 04:15, Lu Baolu wrote:
>>> The hardware assistant vfio mediated device is a use case of iommu
>>> aux-domain. The interactions between vfio/mdev and iommu during mdev
>>> creation and passthr are:
>>>
>>> - Create a group for mdev with iommu_group_alloc();
>>> - Add the device to the group with
>>>          group = iommu_group_alloc();
>>>          if (IS_ERR(group))
>>>                  return PTR_ERR(group);
>>>
>>>          ret = iommu_group_add_device(group, &mdev->dev);
>>>          if (!ret)
>>>                  dev_info(&mdev->dev, "MDEV: group_id = %d\n",
>>>                           iommu_group_id(group));
>>> - Allocate an aux-domain
>>>     iommu_domain_alloc()
>>> - Attach the aux-domain to the physical device from which the mdev is
>>>    created.
>>>     iommu_aux_attach_device()
>>>
>>> In the whole process, an iommu group was allocated for the mdev and an
>>> iommu domain was attached to the group, but the group->domain leaves
>>> NULL. As the result, iommu_get_domain_for_dev() doesn't work anymore.
>>>
>>> This adds iommu_group_get/set_domain() so that group->domain could be
>>> managed whenever a domain is attached or detached through the aux-domain
>>> api's.
>>
>> Letting external callers poke around directly in the internals of
>> iommu_group doesn't look right to me.
>
> Unfortunately, it seems that the vifo iommu abstraction is deeply bound
> to the IOMMU subsystem. We can easily find other examples:
>
> iommu_group_get/set_iommudata()
> iommu_group_get/set_name()
> ...

Sure, but those are ways for users of a group to attach useful
information of their own to it, that doesn't matter to the IOMMU
subsystem itself. The interface you've proposed gives callers rich new
opportunities to fundamentally break correct operation of the API:

dom = iommu_domain_alloc();
iommu_attach_group(dom, grp);
...
iommu_group_set_domain(grp, NULL);
// oops, leaked and can't ever detach properly now

or perhaps:

grp = iommu_group_alloc();
iommu_group_add_device(grp, dev);
iommu_group_set_domain(grp, dom);
...
iommu_detach_group(dom, grp);
// oops, IOMMU driver might not handle this

>> If a regular device is attached to one or more aux domains for PASID
>> use, iommu_get_domain_for_dev() is still going to return the primary
>> domain, so why should it be expected to behave differently for mediated
>
> Unlike the normal device attach, we will encounter two devices when it
> comes to aux-domain.
>
> - Parent physical device - this might be, for example, a PCIe device
> with PASID feature support, hence it is able to tag an unique PASID
> for DMA transfers originated from its subset. The device driver hence
> is able to wrapper this subset into an isolated:
>
> - Mediated device - a fake device created by the device driver mentioned
> above.
>
> Yes. All you mentioned are right for the parent device. But for mediated
> device, iommu_get_domain_for_dev() doesn't work even it has an valid
> iommu_group and iommu_domain.
>
> iommu_get_domain_for_dev() is a necessary interface for device drivers
> which want to support aux-domain. For example,

Only if they want to follow this very specific notion of using made-up
devices and groups to represent aux attachments. Even if a driver
managing its own aux domains entirely privately does create child
devices for them, it's not like it can't keep its domain pointers in
drvdata if it wants to ;)

Let's not conflate the current implementation of vfio_mdev with the
general concepts involved here.

>           struct iommu_domain *domain;
>           struct device *dev = mdev_dev(mdev);
>       unsigned long pasid;
>
>           domain = iommu_get_domain_for_dev(dev);
>           if (!domain)
>                   return -ENODEV;
>
>           pasid = iommu_aux_get_pasid(domain, dev->parent);
>       if (pasid == IOASID_INVALID)
>           return -EINVAL;
>
>       /* Program the device context with the PASID value */
>       ....
>
> Without this fix, iommu_get_domain_for_dev() always returns NULL and the
> device driver has no means to support aux-domain.

So either the IOMMU API itself is missing the ability to do the right
thing internally, or the mdev layer isn't using it appropriately. Either
way, simply punching holes in the API for mdev to hack around its own
mess doesn't seem like the best thing to do.

The initial impression I got was that it's implicitly assumed here that
the mdev itself is attached to exactly one aux domain and nothing else,
at which point I would wonder why it's using aux at all, but are you
saying that in fact no attach happens with the mdev group either way,
only to the parent device?

I'll admit I'm not hugely familiar with any of this, but it seems to me
that the logical flow should be:

- allocate domain
- attach as aux to parent
- retrieve aux domain PASID
- create mdev child based on PASID
- attach mdev to domain (normally)

Of course that might require giving the IOMMU API a proper first-class
notion of mediated devices, such that it knows the mdev represents the
PASID, and can recognise the mdev attach is equivalent to the earlier
parent aux attach so not just blindly hand it down to an IOMMU driver
that's never heard of this new device before. Or perhaps the IOMMU
drivers do their own bookkeeping for the mdev bus, such that they do
handle the attach call, and just validate it internally based on the
associated parent device and PASID. Either way, the inside maintains
self-consistency and from the outside it looks like standard API usage
without nasty hacks.

I'm pretty sure I've heard suggestions of using mediated devices beyond
VFIO (e.g. within the kernel itself), so chances are this is a direction
that we'll have to take at some point anyway.

And, that said, even if people do want an immediate quick fix regardless
of technical debt, I'd still be a lot happier to see
iommu_group_set_domain() lightly respun as iommu_attach_mdev() ;)

Robin.

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