On Tue, Jul 25, 2017 at 12:36 AM, Alex Williamson <alex.williamson@xxxxxxxxxx> wrote: > 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, Yes, per-device iommu_ops will make things much cleaner. That's why I have dropped VFIO no-IOMMU and IOMMU_CAP_BYPASS related patches. Can you please have a look at FlexRM platform reset driver? Regards, Anup
