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. Robin.
