On Thu, 20 Jul 2017 12:17:12 +0100
Robin Murphy [off-list ref] wrote:

On 20/07/17 10:10, Will Deacon wrote:

quoted

On Thu, Jul 20, 2017 at 09:32:00AM +0530, Anup Patel wrote:  

quoted

On Wed, Jul 19, 2017 at 5:23 PM, Will Deacon [off-list ref] wrote:  

quoted

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