On Mon, 24 Jul 2017 18:23:20 +0100 Robin Murphy <robin.murphy@xxxxxxx> wrote: > On 24/07/17 18:16, Alex Williamson wrote: > > On Thu, 20 Jul 2017 12:17:12 +0100 > > Robin Murphy <robin.murphy@xxxxxxx> wrote: > > > >> On 20/07/17 10:10, Will Deacon wrote: > >>> On Thu, Jul 20, 2017 at 09:32:00AM +0530, Anup Patel wrote: > >>>> On Wed, Jul 19, 2017 at 5:23 PM, Will Deacon <will.deacon@xxxxxxx> wrote: > >>>>> There are two things here: > >>>>> > >>>>> 1. iommu_present() is pretty useless, because it applies to a "bus" which > >>>>> doesn't actually tell you what you need to know for things like the > >>>>> platform_bus, where some masters might be upstream of an SMMU and > >>>>> others might not be. > >>>> > >>>> I agree with you. The iommu_present() check in vfio_iommu_group_get() > >>>> is not much useful. We only reach line which checks iommu_present() > >>>> when iommu_group_get() returns NULL for given "struct device *". If there > >>>> is no IOMMU group for a "struct device *" then it means there is no IOMMU > >>>> HW doing translations for such device. > >>>> > >>>> If we drop the iommu_present() check (due to above reasons) in > >>>> vfio_iommu_group_get() then we don't require the IOMMU_CAP_BYPASS > >>>> and we can happily drop PATCH1, PATCH2, and PATCH3. > >>>> > >>>> I will remove the iommu_present() check in vfio_iommu_group_get() > >>>> because it is only comes into actions when VFIO_NOIOMMU is > >>>> enabled. This will also help us drop PATCH1-to-PATCH3. > >>> > >>> I don't think that's the right answer. Whilst iommu_present has obvious > >>> shortcomings, its intention is clear: it should tell you whether a given > >>> *device* is upstream of an IOMMU. So the right fix is to make this > >>> per-device, instead of per-bus. Removing it altogether is worse than leaving > >>> it like it is. > >> > >> Not really - if there is an IOMMU up and running to the point of setting > >> bus ops, every device it cares about can be expected to have a group > >> already (there are only a couple of drivers left that don't use groups, > >> and they're hardly relevant to VFIO). Thus iommu_group_get() already is > >> the de-facto per-device IOMMU check. > >> > >> And having looked into it, I'm now spinning a couple of patches to > >> finish off making groups truly mandatory so that that can be less > >> de-facto ;) > > > > No, look at vfio-noiommu and even vfio-mdev devices for devices which > > have an iommu group but there is no physical iommu supporting them. > > iommu_present() is how we can distinguish these groups and therefore > > not generate a segfault in trying to use the full IOMMU API on them. > > OK, so that means that the combination of vfio-noiommu and vfio-platform > is simply unusable, because iommu_present(&platform_bus_type) can give > such dangerous false positives too. Yep, this kinda falls apart since platform_bus_type doesn't really map to a physical bus, nor does the presence of a group canonically demonstrate that an iommu is present since anyone can create a group for a device. We really do need to migrate to per-device iommu_ops. Thanks, Alex
