Re: [PATCH 1/2] iommu: Add iommu_group_get/set_domain()

[Lu Baolu]

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
> > > > ÂÂÂÂÂÂÂÂ group = iommu_group_alloc();
> > > > ÂÂÂÂÂÂÂÂ if (IS_ERR(group))
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return PTR_ERR(group);
> > > >
> > > > ÂÂÂÂÂÂÂÂ ret = iommu_group_add_device(group, &mdev->dev);
> > > > ÂÂÂÂÂÂÂÂ if (!ret)
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ dev_info(&mdev->dev, "MDEV: group_id = %d\n",
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ iommu_group_id(group));
> > > > - Allocate an aux-domain
> > > > ÂÂÂÂiommu_domain_alloc()
> > > > - Attach the aux-domain to the physical device from which the mdev is
> > > > ÂÂ created.
> > > > ÂÂÂÂiommu_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:
>
>  ÂÂÂÂdom = iommu_domain_alloc();
>  ÂÂÂÂiommu_attach_group(dom, grp);
>  ÂÂÂÂ...
>  ÂÂÂÂiommu_group_set_domain(grp, NULL);
>  ÂÂÂÂ// oops, leaked and can't ever detach properly now
>
> or perhaps:
>
>  ÂÂÂÂgrp = iommu_group_alloc();
>  ÂÂÂÂiommu_group_add_device(grp, dev);
>  ÂÂÂÂiommu_group_set_domain(grp, dom);
>  ÂÂÂÂ...
>  ÂÂÂÂiommu_detach_group(dom, grp);
>  ÂÂÂÂ// 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.
>
> > ÂÂÂÂÂÂÂÂÂÂ struct iommu_domain *domain;
> > ÂÂÂÂÂÂÂÂÂÂ struct device *dev = mdev_dev(mdev);
> > ÂÂÂÂÂÂ unsigned long pasid;
> >
> > ÂÂÂÂÂÂÂÂÂÂ domain = iommu_get_domain_for_dev(dev);
> > ÂÂÂÂÂÂÂÂÂÂ if (!domain)
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ return -ENODEV;
> >
> > ÂÂÂÂÂÂÂÂÂÂ pasid = iommu_aux_get_pasid(domain, dev->parent);
> > ÂÂÂÂÂÂ if (pasid == IOASID_INVALID)
> > ÂÂÂÂÂÂÂÂÂÂ return -EINVAL;
> >
> > ÂÂÂÂÂÂ /* Program the device context with the PASID value */
> > ÂÂÂÂÂÂ ....
> >
> > 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:
>
>  ÂÂÂÂ- allocate domain
>  ÂÂÂÂ- attach as aux to parent
>  ÂÂÂÂ- retrieve aux domain PASID
>  ÂÂÂÂ- create mdev child based on PASID
>  ÂÂÂÂ- 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