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

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.

quoted

  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