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. Thanks, Alex
