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 ;) Robin. >>> 2. If a master *is* upstream of an IOMMU and you want to use no-IOMMU, >>> then the VFIO no-IOMMU code needs to be extended so that it creates >>> an IDENTITY domain on that IOMMU. >> >> The VFIO no-IOMMU mode is equivalent to Linux UIO hence having >> IDENTITY domain for VFIO no-IOMMU is not appropriate here. > > Can you elaborate on this please? I don't understand the argument you're > making. It's like saying "I don't like eggs, therefore I don't drive a > car". > > Will >
