Messages in this thread | | | Subject | Re: [PATCH 1/2] iommu: Add iommu_group_get/set_domain() | From | Lu Baolu <> | Date | Tue, 7 Jul 2020 09:26:48 +0800 |
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On 7/2/20 10:36 AM, Lu Baolu wrote: > Hi Robin, > > On 7/1/20 8:18 PM, Robin Murphy wrote: >> On 2020-07-01 08:32, Lu Baolu wrote: >>> 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? >> >> Rather than pass a bare iommu_group with implicit restrictions, it >> might be neater to just pass an mdev_device, so that the IOMMU core >> can also take care of allocating and setting up the group. Then we >> flag the group internally as a special "mdev group" such that we can >> prevent callers from subsequently trying to add/remove devices or >> attach/detach its domain directly. That seems like it would make a >> pretty straightforward and robust API extension, as long as the mdev >> argument here is optional so that SVA and other aux users don't have >> to care. Other than the slightly different ordering where caller would >> have to allocate the mdev first, then finish it's PASID-based >> configuration afterwards, I guess it's not far off what I was thinking >> yesterday :) > > It looks good to me if we pass an *optional* made-up device instead of > iommu_group. But it seems that vfio/mdev assumes an iommu_group first > and then attaches domains to the groups. Hence, it's hard to move the > group allocation and setting up into the attach interface. > > As proposed, the new iommu_aux_attach_device() might look like this: > > int iommu_aux_attach_device(struct iommu_domain *domain, > struct device *phys_dev, > struct device *dev) > > where, > > @phys_dev: The physical device which supports IOMMU_DEV_FEAT_AUX; > @dev: a made-up device which presents the subset resources binding to > the aux-domain. An example use case is vfio/mdev. For cases where > no made-up devices are used, pass NULL instead. > > With @dev passed, we can require > > - single device in group; > - no previous attaching; > - set up internal logistics between group and domain; > > The iommu_aux_detach_device() needs the equivalent extensions.
Okay, let me send out the code first so that people can comment on the code.
Best regards, baolu
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