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. > > 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
