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SubjectRe: [PATCH 1/2] iommu: Add iommu_group_get/set_domain()
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Hi Robin,

On 2020/7/1 0:51, Robin Murphy wrote:
> 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() ;)

Get your point and I agree with your concerns.

To maintain the relationship between mdev's iommu_group and
iommu_domain, how about extending below existing aux_attach api

int iommu_aux_attach_device(struct iommu_domain *domain,
struct device *dev)

by adding the mdev's iommu_group?

int iommu_aux_attach_device(struct iommu_domain *domain,
struct device *dev,
struct iommu_group *group)

And, in iommu_aux_attach_device(), we require,
- @group only has a single device;
- @group hasn't been attached by any devices;
- Set the @domain to @group

Just like what we've done in iommu_attach_device().

Any thoughts?

Best regards,
baolu

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