From: Lu Baolu <firstname.lastname@example.org> To: Robin Murphy <email@example.com>, Joerg Roedel <firstname.lastname@example.org>, Alex Williamson <email@example.com> Cc: Kevin Tian <firstname.lastname@example.org>, Dave Jiang <email@example.com>, Ashok Raj <firstname.lastname@example.org>, email@example.com, Cornelia Huck <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org Subject: Re: [PATCH 1/2] iommu: Add iommu_group_get/set_domain() Date: Wed, 1 Jul 2020 15:32:22 +0800 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <firstname.lastname@example.org> 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 _______________________________________________ iommu mailing list email@example.com https://lists.linuxfoundation.org/mailman/listinfo/iommu
next prev parent reply other threads:[~2020-07-01 7:32 UTC|newest] Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-06-27 3:15 Lu Baolu 2020-06-27 3:15 ` [PATCH 2/2] vfio/type1: Update group->domain after aux attach and detach Lu Baolu 2020-06-29 11:56 ` [PATCH 1/2] iommu: Add iommu_group_get/set_domain() Robin Murphy 2020-06-30 1:03 ` Lu Baolu 2020-06-30 16:51 ` Robin Murphy 2020-07-01 7:32 ` Lu Baolu [this message] 2020-07-01 12:18 ` Robin Murphy 2020-07-02 1:32 ` Lu Baolu 2020-07-02 2:36 ` Lu Baolu 2020-07-07 1:26 ` Lu Baolu
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